It's very simple reason why how it can be explained...when Clara jumped into the doctors time line ..what happened to her? Multiplied into many Clara's in the Doctors very own time line...she merged with the very Doctors time line ...what did the doctor do..jumped in after her..
By doing such it multiplied the Doctor throughout his very own timeline...and since each duplicated Doctor also Regenerates....infinite numbers of the Doctor..
And since the very core ability of all Time Lords is the ability to regenerate...there is bound to be variable Regenerations of each time duplicated Time Lord..
http://www.oocities.org/sege1701/TimeLord/files/tlcomp.zip
Take a read on those books ian marsh & Peter were involved in writing novels for the new adventures of Doctor who during the 90's and those were about the 7th incarnation of the Doctor..
Remember the 12 th talked about the corsair's ability to regenerate into the opposite sex..bit in hell bent...the general regeneration into female..
Regeneration is random most of the time unless your like the master which has a pretty good control over his regeneration process.
I AM BOLO
100% lover "nothing else matter's" after that...
ps:
Proud psOne/2/3/p owner. I survived Aplcalyps3 and all I got was this lousy Signature.