All Pokemon (as a species) are split into 6 categories of experience growth.
This is usually recognized by how much exp is needed to reach lv 100.
Erratic is the fastest growth rate meaning it needs the least exp to get to lv 100 (600,000).
On the other hand Fluctuating is the slowest growth rate. (1,640,000 at lv 100)
Look up this link here as you may find it useful.
Though Erratic is the fastest growth rate at early lvs it is actually the slowest and are usually the harder Pokemon to lv up. Where as the slowest Fluctuating is the quickest group of Pokemon to lv up in their early stages.