Lots of explanations. I'll make some up:
You didn't say when it was paid, just that you noticed it now. Maybe look more often?
Date of Record was months ago, but Payable Date was this week.
Vanguard should have paid you months ago and just found the money.
VBS is a poor excuse for a brokerage.
It's all about market timing, uh, I mean rebalancing, uh, I mean opportunistic rebalancing, uh, I mean short-term opportunistic rebalancing due to a short-term change in one's asset allocation.