Because the shorter shots interact primarily with the cover, while the longer shots interact primarily with the core.
It's like in table tennis you can have a fast rubber that spins a lot, a fast rubber that doesn't spin much, a slow rubber that spins a lot, or a slow rubber that doesn't spin much.
Speed comes from the core (the layers and thickness and the padding, whether you've speed glued it, etc.) while spin comes from the tackiness and softness of the outer layer… because when you spin the ball you're giving it a glancing blow, and so you don't compress down into the lower layers (or core), just like a golf ball.
Slower shots with more loft or AoA relative to the face rely on the cover, while faster swings with more straight faces rely on the core or lower levels of the rubber/padding.
I've heard I can't practice on mats, mats are no good. No, you're just not practicing and learning correctly. You can find ways to sense/see you're hitting fat, video being one of them. Or you're just looking for an excuse to pay for extra for that grass range, but your still practicing badly.