My understanding of how the math works out is that PCs get a pretty significant boost in effectiveness as soon as they get into Paragon Tier. So I do think that's a factor.
If you're using standard encounter budgets, I'd guess that a lot of those monsters come from high heroic or just into paragon, and the monsters right on either side of that split seem to have a noticeable difference in effectiveness.
There are some low-paragon creatures that are just way deadlier (such as the ghaele of winter), even though they're not many levels higher. So you might fiddle with monster composition a bit (pulling maybe smaller encounters that show some paragon 'oomph' and synergize well), and see how that goes.
Sometimes it's just a matter of the players coming up with tactics that largely neutralize the nastiest part of an encounter, which is cool, since they're getting a payoff for their efforts. So unless they seem like they're not trying as hard because it's too easy, you may not have to bump difficulty much, it may be more a matter of the back-and-forth between you and the players tactically.
If you're using standard encounter budgets, I'd guess that a lot of those monsters come from high heroic or just into paragon, and the monsters right on either side of that split seem to have a noticeable difference in effectiveness.
There are some low-paragon creatures that are just way deadlier (such as the ghaele of winter), even though they're not many levels higher. So you might fiddle with monster composition a bit (pulling maybe smaller encounters that show some paragon 'oomph' and synergize well), and see how that goes.
Sometimes it's just a matter of the players coming up with tactics that largely neutralize the nastiest part of an encounter, which is cool, since they're getting a payoff for their efforts. So unless they seem like they're not trying as hard because it's too easy, you may not have to bump difficulty much, it may be more a matter of the back-and-forth between you and the players tactically.