As you train, equip, and fuse your Crew, their Skills will improve. The best way to get a sense of how skilled your Crew will be at higher levels is to look them up on the Star Trek Timelines Wiki, which is maintained by the community and found here:
It is important to note that just because something is a “primary” skill, it doesn't necessarily mean that it will be their best. It all comes down to how the crew was designed; for example, a crew may be designed to be better than average or average in their primary skill for the first 20 levels or so, but even out or even get significantly below average by their higher levels. That same crew might have a secondary or tertiary that isn't great early, but develop to be much higher than average by lvl 100. These are all design decisions made in accordance with what best fits the feel of the crew/needs of the game.