I think there are a few reasons why draft and bottled beer taste different and some of the reasons may recolor your view of draft beer. Some breweries actually have different variations of their beers for draft and bottle. I do not have real good information on how prevalent this is, but the examples I am aware of have two commonalities: the draft beer has a lower bitterness and lower carbonation compared to the bottled beer. Sierra Nevada Pale Ale draft beer is different from its bottled cousin in that the draft form is a little darker and has a lower original gravity compared to the bottled version. Both beers are excellent. I assume that their draft Pale Ale is intended to be more of a session beer and the difference in the recipe makes it more quaffable. So this could explain your observation.
Other factors that make draft beer taste different from the bottled can make a brewer an unhappy camper. Some bars put their beers on “beer gas,” also called mixed gas, to “smooth” out beer flavor. An unintended outcome of this practice is that these beers lose their carbonation and deviate from the brewery specification. This really irks me because if a brewer decides they want to serve their draft hefeweizen at three volumes of carbon dioxide, the bar owner or distributor has no business doing something that changes the character of the beer.
In an effort to lower operating costs, some bars use dreadful contraptions called air blenders. This cheap way of making mixed gas creates a mixture of carbon dioxide and air, which replaces nitrogen with the more affordable compressed air. The use of air blenders flatten kegs over time. They also pump oxygen into the keg to oxidize the beer. Sometimes the air compressor hooked to the blender introduces a bit of microbiological wildlife and whatever funky smells are next to its intake into the mix of carbon dioxide, nitrogen, oxygen. In short, air blenders can change the flavor of kegged beer in a variety of different and disappointing ways.
The use of mixed gas and air blenders make bar owners really happy because they reduce beer carbonation, making the beer easier to pour. This practice reduces beer loss caused by the sloppy bartenders slinging pints behind bars scattered across this great land. If I sound a bit harsh towards bar owners and bartenders it’s because that is my intent! The reason that beer advocacy and travel magazines make such a big deal about great draft beer bars is because of the astounding number of really awful draft bars.
Another interesting factoid about draft beer is the flexible line linking the keg to the tap. These little buggers can turn into veritable small intestines. In other words, with their relatively large surface to volume ratio and their tendency to become covered in a microbiological film when neglected, draft lines can turn into long, thin bioreactors that change the flavor of beer as it flows from keg to tap. Neglected taps can also become totally funkified with microbiological growth. This can become especially pronounced with unfiltered beers as the nooks and crannies of the beer tap can quite literally become coated with a visible film of living yeast.
The key to draft beer is really quite simple. Carbon dioxide pressure and beer storage temperature should be matched to the carbonation level of the typical draft beer (usually somewhere around 2.5 volumes of carbon dioxide), and draft lines need to be routinely cleaned. In well-run draft bars, the flavor of draft beer should be within the expectations of the brewery and any difference between a draft and bottled beer should be minimal unless the beer has different draft and bottle specifications.
You mention that you perceive draft beer to taste smoother than bottled beer. This may come from the difference in pouring techniques between the two. It is often the case with draft beer that a noticeable amount of carbon dioxide is “knocked” out of the beer during pouring. In contrast, bottles are easier to gently pour and the amount of carbon dioxide loss is much less. Since carbonation level influences perceived bitterness, any differences in carbon dioxide content between draft and bottle may also lead to apparent differences in bitterness.
As a brewer I prefer draft beer because it is less labor intensive to produce than bottled beer. Recognizing the factors that influence flavor you can adjust your recipe, carbonation level and serving temperature to produce the beer that you want, which is really what homebrewing is all about. And with the proper care and maintenance of your draft system you can avoid many of the problems that readily arise when draft neglect occurs!