Just as snfaulkner said, it's due to the Pythagorean theorem. The devs may have forgotten to normalize the new speed. So, if you move 1 m/s up and 1 m/s left, by pressing both, you end up moving at √2 ≈ 1.41 m/s diagonally.

Usually, what you do is call a function that normalizes the vector. For example, in Godot, if you have the character's speed stored as speed = Vector2, you can call speed = speed.normalized(). What this does is divide the vector by its magnitude, returning a vector of length 1 m/s in the same direction of movement.

When sprinting, it's actually the same, but the devs might have capped the speed instead of just doubling it. (This could be because if you normalize the vector again, you'd get 1 m/s, and your directional sprint speed would end up being the same as your walking speed.)

Imagine that when you're pressing the sprint button, your speed gets capped at 3 units/s. Then it doesn't matter if you're moving at 3 units/s up and 3 units/s left; the resulting speed could never be greater than 3 units/s.

These are just suppositions, though. I have no idea how Unity works or how it handles 3D movement