the pouring way of beer won't cause such big difference between bottled and tap Guiness, it's just because they are different.
If you know the basic way of making beer,
6.Cooling or Chilling, Yeast Pitch
before the step 9, there is filtration and heating for beer will be bottled in bottles and cans, but only filtation (maybe no neither) for tap beer. that's make the difference.
I wish if it could help you