Here's a few questions, which may or may not shed light on the subject:
1. In the TCG, there are different versions of the same pokemon, i.e. different versions of Pikachu. Do they all have the same level?
2. When a pokemon evolves to the next stage in the TCG, i.e. Pikachu to Raichu, does Raichu always have the same level?
If not, it would imply to me that evolving (in the TCG) doesn't necessarily go one level at a time, and can be any number of levels difference. By that logic, wouldn't it make sense that "leveling up" in the TCG doesn't mean increasing by just one level, but it could be some unknown quantity of levels?