Just a follow-up on Halls' comment:
You probably know many card games, each one having its own specific rules as to what valid "moves" are.
Now, do you regard one such set of rules to be the only valid way to pass the time with a deck of cards?
Similarly with maths:
With different sets of ground rules for what is "valid" you get different types of math games.
You can't say in general that one such set of rules is "truer" than any other such set.
However, some particular sets of rules might be regarded as "dumb":
For example, consider a card game that in some situation said both that a particular "move" is permitted, and in the same breath said that that move was forbidden. That is, the set of rules is self-contradictory, and hence dumb.
Such system weaknesses ought of course be avoided, but in complicated logical systems, it is not always apparent whether or not a particular set of rules can generate internal contradictions or not.
But this feature does not in any way make it more (or less) difficult to ascertain whether a particular deduction in a particular system is valid or not, it only makes it more difficult to ascertain whether the particular set of rules used is a good or bad idea in the first place.