I imagine that with glass changes, improved coatings, changing production processes, and possibly the beginning of the use of computers for determining optical formula, there were companies that revisited old lens formulas. As Nitroplait said, the Tessar in the T is supposed to be a reformulated improved lens. I can't say for certain, as for me this is just one of those 'oh yeah everyone knows....' internet things.
Then again, at this time lens design was moving far beyond 4 element designs, with assorted 6 and more elements becoming the norm with coatings and more precise machining and such. So reformulating an outdated design seems a bit silly. Not sure what motivated Zeiss with the Tessar (if that is true) but have not heard of the Xenar undergoing this until now.