<= vs =<

What's the difference between these two operators? I could not find any definition for the former (even though it looks similar to => which is used for logical implication).

I had mistakenly typed <= for less than or equal in the definition of SmallToBig while going through video 4 (Die Hard) of the video lecture series.

The parser and model checkers did not complain: I even got a valid counter-example for the invariant big#4. However, the behaviour of the model checker using '<=' seems to be different. It explores a different number of states, and the state-space has a different diameter.