Hmm, i found the maybe best way to explain number discrepancies is to assume that witches and familiars are responsible for the majority of non peaceful death worldwide, and maybe a majority of overall suffering. Assuming they dont have to be blatant, kisses can influence people more subtly, and/or they can influence and/or feed on such stuff just from being nearby... If we place the majority of every murder, suicide, violent death and maybe other hard crime as being their responsibility (and thus feeding into the deaths required) may make the numbers more realistic.
Anyway, if we take the average lifesapn of two years, then far, far majority of witches will be grown familiars. Leaving fringe cases like disconnected souldgems aside for now, if the average mg kills one witch per week, lasts for two years, and all always witch out and never die in other ways, that still leaves you with 1 in a 100 for "original" witches. Bumping it down to two weeks still makes it 1 in 50, and factoring in a that many PMs die without witching would only drive the number up again.
On the other hand, if soulgems disconnected can last indefinitely, whats the chance that there are some gems buried somewhere or on the bottom of the ocen, for centuries or milennia, driving up the average?