Why can you run over 1 block gaps sometimes but you can't on other times?

I’ve known the fact that you can run over 1 block gaps sometimes but you cannot on other times for as long as I’ve owned the game. However, why can’t I do it all the times? When I say "run", I mean just sprinting, no speed potions or walking on ice blocks, just running on normal blocks(eg. Stone, cobblestone, oak wood planks). Can someone please tell me why? Its just kind of weird for me. Thank you.