You see people talk about a 60 IBU beer where the IBUs all come from big doses of late hops being "smoother" than 60-min hop IBUs. This always strikes me as odd. If its smoother then its not as bitter, and therefore should have a lower IBU number. What they're really saying is that the equations say both beers are 60 IBUs, which they can't be the same, by simple inspection with your tongue! More evidence that the equations are flawed. If someone wants to argue that there's different kinds of bitterness, then the equations are even less useful.
Actual bitterness and perceived bitterness are very different. A boil start IBU of 68 will come off as drastically different vs. a boil start IBU of 23, with the 10 minute and hopstand additions taking care of the rest (even if both beers end up with the same final IBUs).
Perceived bitterness can come from excess tannins, stale ingredients, long dryhop times, poor yeast practices, poor hop selections, an imbalanced recipe, etc.
Example 1:
10# 2-Row
1# White Wheat
1/2# C-20
1/2# Corn Sugar
68 IBUs
1.00 oz. Apollo 20% @ 60 minutes
2.00 oz. Centennial sub 165 Hopstand (no isomerization)
2.00 oz. Cascade sub 165F Hopstand (no isomerization)
2.00 oz. Centennial 14 Day Dryhop
2.00 oz. Amarillo 14 Day Dryhop
Example 2:
68 IBUs
0.33 oz. Apollo 20% @ 60 minutes
2.00 oz. Centennial 10% @ 10 minutes
2.00 oz. Cascade 7% @ 180F Hopstand (10% Utilization)
0.33 oz. Apollo @ sub 165 F Hopstand (no isomerization)
2.00 oz. Centennial 5 Day Dryhop
2.00 oz. Amarillo 5 Day Dryhop
0.33 oz. Apollo 5 Day Dryhop
^ Same amount of IBUs, same amount of hops, same type of hops, very similar hop bills, yet Example 2 will come off as less bitter. You will sense more juicy-ness as a result of having more beneficial hop compounds/oils. And that juicy-ness is perceived as slightly sweeter.