What PetertheAleut said is common practice if a priest starts to run low on wine. One caveat: In seminary, we were taught to add extra wine BEFORE you totally ran out. That way, the wine could co-mingle. So, I would say the most natural theological explanation (if there is any!) is that one can't really distinguish between the previously consecrated molecules of wine and the newly added -- they're all mixed together -- so it's all good. Or, you could say, like PetertheAleut, that the new stuff gets blessed by association. (Let's leave it to scholastics to figure out which is the best theological explanation.)
If one starts to run low on bread, one just starts giving out tiny, tiny little specs of crumbs. Pretty easy to control portion size with the spoon. If the priest starts running low on mini-crumbs, then he can always go back into the altar, take some of the reserve sacrament (already consecrated dried bread in the Tabernacle on the Holy Table), and add that to the chalice. Seen that done a dozen or so times.