The phrase often relates to sports performance but applies to other activities too.

Sports Origin

It suggests a temporary decline rather than a permanent inability.

Temporary State

Being 'off one's game' is subjective; what's off for one may be normal for another.

Indicates a performance that is better than the individual's average level.

The phrase can imply that the high performance seems effortless or natural.

Often used to compliment someone who consistently performs well.