There are a couple explanations, mostly related to relative MMR values, gains and losses.
Particularly if your second account has never played ranked before, as your first 50 lifetime ranked matches apply bonus MMR rewards, the intention being to more quickly sees you at an accurate ranking.
In short, getting lucky and getting paired more often against higher aggregate MMR teams while maintaining a decent win clip can artificially inflate your MMR slightly, potentially leading to a faster climb.
In general, MMR inflation is one of the most important factors in regards to how long it will take one to make a certain climb, the greater the MMR+ inflation (MMR over division average) the faster the climb.