B-b,
Don't like to guess or answer a question with more questions. Normally just pass when there is not enough info to make a call. Since you mention reversal, will assume you wintered in double deeps, but it would help to have more data on the early season steps.
Like which box had the basic broodnest when reversed.
As to why you are seeing more positive backfilling than normal, consider how the spring season unfolded. The extra cold late winter retarded developement of both the trees and bees. Low growing stuff, closer to the big heat sink (earth) were not retarded as much. As a result, the "flow" was compacted to a shorter and stronger period. A large number of their favorite sources were blooming in parallel. The bees had the advantage of picking the best of the best.
I may be the only person who believes that the bees have an uncanny skill at reading the season as it unfolds. This opinion stems from the the fact that all colonies in a given location invoke reproduction cut off within a few days of each other, regardless of race or status.
So, this season the bees are behind calendar schedule and presently, there is a crush of forage available. It would not surprise me if they were merely making up for lost time. They have a lot of Plan Bs or work-arounds in their bag of tricks. And swarming is the primary motivation of late winter/early spring.
Walt