It is entirely possible that the new lens is sharper. Newer production line, newer technology, newer coatings, even a revised design. These all may contribute to a better lens.
However, you may have got one that slipped by quality assurance. No company hits 100%. An inspector may be secretly nursing an hangover, planning the weekend or about to move to a new job. Same as in any company. People screw up and the company takes the blame.
If it is any consolation, I hear from Leica owners that the highly vaunted quality of the Leitz lenses may have sliped, letting quite a few really nasty lenses through as well. There has been speculation that since Leitz sells a high percentage of their cameras to collectors who *NEVER* shoot, the lenses can slip along on their old reputations.
I have never found a camera store that did not allow me to put a lens on my camera body, and shoot some tests in the store and out the door. I have never had to put up a deposit to hold the lens either, but one may reasonably be asked to. Process and print the film immediately and check it. If the lens is sharp, buy it right away. Nothing beats a good relationship with a camera store.
Make sure you note down the serial number, since lenses do vary from one to the next. Buying a different serial number certainly invalidates the test.
Realize also that you have a manufacturer's warrantee and good camera stores have a return or exchange policy. If you get a dud, you are not necessarily stuck with it.
larry!
