The reason for this is because Hotslogs only shows winrates for hero levels 5 and up by default. You're far more likely to see level <5 Galls than Chos in hero league. In addition, if you're looking at a specific league, that will only show the winrate from players in that league, even when they played in games that included lower-ranked players. Again, if you have a Cho&Gall duo, it is likely the more skilled player plays Cho.