PioSolver wrote a good answer to this question in their FAQ, I'll paste it here:
Why do so many options, bet/check, raise/call, so frequently have basically the exact same EV?
Quote:
This is one of the biggest surprises about gto solutions. It turns out that especially on the flop there aren't really a clear bets or very few of them. Usually the differences are quite clear between calls and folds (at least for most hands) but check/bet decisions and often call/raise ones turn out to be close.
It doesn't mean you can just start checking all those mixed hands or betting them as that would open a huge opportunity for the opponent to exploit you. This means it's a good idea to focus on general range composition for actions but not necessirily exact hands and frequencies.
Here's my take:
There are effectively an infinite number of strategies, and some subset of those satisfy the exploitability requirements of our solution. It should come as no surprise, especially in spots where there's a lot of game tree left, that there are many ways to play.