There can be many factors in this -
1. Leveling up your character after you have achieved required points is NOT mandatory (you can stock up points for quick level up at your choice). So theoretically you can have any level 25 Girl(s) even at Player level 1 - provided you have resources for it (Girl cards and Gold).
2. Girl damage can be increased from Sex pills, Toys ( instead of balanced distribution of Viagra, you do a skewed level up in favor of lets say , Player HP and WATER girls damage - so you can have player with more damage and HP even at same level). Player who has better Toys (Mythic or Legendary) will have better damage increase for selected girl.
3. Cheating (this is obviously easily to digest and assume BUT it may not be the case always)