OP, couple things:

The trigger for that fan on signal is going to be heat, not wattage. Granted those things usually go hand in hand but there are a handful of reasons that could be responsible for the behavior you’re seeing:

• Your PC is exhibiting an unusually high draw on one depreciated rail and pushing that part of the PSU near it’s limit. Legacy components can be good for this, -12V is a great example of a depreciated power supply voltage.

• You’re seeing heat from something else soaking into the PSU. GPU is a good internal example. If you’re an audiophile, headphone amp is a good external one especially if it’s sitting on the tower.

• PSUs are designed to allow other case ventilation to pass through them for cooling. You’ve probably got another non-PSU cooling issue - clogged filter, dead or dying fan, negative pressure.

Lastly, it’s hard for us to say whether that power draw is appropriate without knowing more about your PC. In general, for a desktop with a dedicated GPU it’s in the ballpark but again hard to nail down specifics. It’s a big ballpark. Things like what idle states are supported and enabled can easily account for 5W.