As others have said, it's something you can adjust if you feel is right for your campaign.
I think they did it to
1) keep pacing so that the 'default' has different types of monsters at different ranges
2) to keep somethings a bit more iconic in power level (i.e think way back how dragons were never low level so they had more fear and majesty about them, no one wants to cross one! versus a skeleton, every 1st level adventurerer runs across them, so they don't have as much fear. with a vampire, they probably want it to be in the mid range where they aren't that common but still a manageable threat
3) powers... some monster powers are just kept at certain levels. for instance, i don't think anything lower than a level 7 monster will ever stun. vampires have a charm effect so there is probably a minimum that that sort of thing is considered 'ok'
4) .. eh, there is more but i'm too tired now to keep typing.
but all this goes back to the original point -- if you want to shake up the frequency and power of various monsters for your game, by all means. go for it (i do it all the time). it keeps people on their toes when it's not always what they expect.
I think they did it to
1) keep pacing so that the 'default' has different types of monsters at different ranges
2) to keep somethings a bit more iconic in power level (i.e think way back how dragons were never low level so they had more fear and majesty about them, no one wants to cross one! versus a skeleton, every 1st level adventurerer runs across them, so they don't have as much fear. with a vampire, they probably want it to be in the mid range where they aren't that common but still a manageable threat
3) powers... some monster powers are just kept at certain levels. for instance, i don't think anything lower than a level 7 monster will ever stun. vampires have a charm effect so there is probably a minimum that that sort of thing is considered 'ok'
4) .. eh, there is more but i'm too tired now to keep typing.
but all this goes back to the original point -- if you want to shake up the frequency and power of various monsters for your game, by all means. go for it (i do it all the time). it keeps people on their toes when it's not always what they expect.
