Just to be clear... this isn't the seller's fault in any way, it's o-like who deserves the blame.
The laser is advertised as being >350mW, even though they label it 400mW.
Also unlike most hobbyists o-like only takes peak and raw optical reading into account. So for example the laser peaks at 317mW and actually puts out stable ~300mW. If I were to sell it I would put it as 300mW.
The waterproof and focusing lens also take away from the total output.
So according to o-like's convoluted logic this laser is actually on spec.
Needless to say, I don't agree and think that portion of their approach is BS. Unfortunately it's still better than the other 99% of chinese companies that just guess at the output.