I don't know the exact etymology of this word in Celtic - just mentioning some other languages with a similar relationship to PIE, but not necessarily with the same degree of influence by Latin, for comparison.
I'm not sure what you mean by "mutation of the words happened identically over time"? The Latin for "ten" is "decem" - it didn't drop the "m" (hence also "decimus", "decimatio" etc). Most Romance languages did drop it eventually - but that came later, and in any case those languages wouldn't be in a position to influence Welsh.
FWIW, I don't think it would be all that surprising for related but diverged languages to evolve in a similar way. Sure, they may have developed some different laws governing those changes since the divergence - but they will also have some common laws from their shared ancestry. And there's "convergent evolution" of languages, as well. Although it would be interesting to find out if this particular change is really governed by some common rule, or some rule that independently arose in two different branches, or it's just a curious coincidence.
I'm not sure what you mean by "mutation of the words happened identically over time"? The Latin for "ten" is "decem" - it didn't drop the "m" (hence also "decimus", "decimatio" etc). Most Romance languages did drop it eventually - but that came later, and in any case those languages wouldn't be in a position to influence Welsh.
FWIW, I don't think it would be all that surprising for related but diverged languages to evolve in a similar way. Sure, they may have developed some different laws governing those changes since the divergence - but they will also have some common laws from their shared ancestry. And there's "convergent evolution" of languages, as well. Although it would be interesting to find out if this particular change is really governed by some common rule, or some rule that independently arose in two different branches, or it's just a curious coincidence.