Look at some of the big wins and losses that you see/experience - when 2 people make strong hands. Good players will tend to lose less by recognising when they're against an absolute monster, but when 2 players make monster hands, sometimes it's just luck as to who has the better monster, and that can create significant swings up and down.
If you look at stats then in the long run the luck evens out, but you're talking periods of months for professional players... as you look at shorter and shorter terms, the chance of some lucky or unlucky hands being a significant factor in your profitability increases. If you join a table and play one hand, your skill generally won't determine whether you make a profit on that hand - how the cards fall will. Over 10 hands, it's mostly luck. Over 100 hands, skill should be more evident, but luck can easily more that cover for any ongoing progression from skill. 1000 hands, then skill will generally be starting to show, but even then a serious hot or cold streak can be the dominant factor. Only at, say, 10,000 hands or more should you see skill becoming most likely the dominant factor, as the short-term luck balances out.
