Thank you for explaining this.It has already been explained that the gearing/size/pitch of the prop is critical. (For example, most 5hp or so outboards are expected to be driving small lightweight dinghies and ribs, but lower geared/propped versions (often with long shafts) are also sold for use driving small yachts.)
An outboard which has an over-pitched/geared prop for the particular boat & load may never reach the revs at which it develops its maximum power, and be beaten by an outboard with lower rated power which can.
Conversely, an under-pitched prop may exceed its peak power revs before a speed which a higher 'geared' but lower powered outboard can reach.
In car terms, a lower powered car may easily beat a higher powered car up a hill if the latter is labouring in top gear while the lower powered one has an appropriate lower gear selected, or if the higher powered one is stuck in first gear.
If you can actually tell the difference between those speeds and that is important to you, then simply buy the fastest one and don't worry about the rated output.