Four collection deletes and score drops

[nathan]

I disputed all derogs on my report (about 12). Ex deletes four of them and my score drops 15 points. is this because I lost history even though they were derogs? I was expecting a atleast a slight jump.

[Anonymous]

What kind of account?  How long with them? 

I would guess that if they deleted the entire account and scores dropped, it would be due to age, but that just MO.