It may not have been so well-cellared after all and possibly kept in such a dry area that it had some evaporation. At almost 20 years old that is a good possibility.
If it was originally filled like that, it shouldn't be a problem, as the bottles are sparged with an inert gas on the bottling line to remove oxygen right before being filled. So that empty space should not have much if any oxygen there. I'd open one up and see how it tastes....that's the only real way to check.
It may not have been so well-cellared after all and possibly kept in such a dry area that it had some evaporation. At almost 20 years old that is a good possibility.
There is some damp engendered label degradation, especially on the bottles at the bottom of the case, and the original shipping labels on the outside of the case have been slug chewed; so I think that's unlikely. It's also been in my cellar for over two years, which normally has a humidity around the 80% mark.
When bottles in a case lose level, it doesn't happen uniformly; there are always some that are worse than others..
Tom
I may be drunk, Miss, but in the morning I shall be sober and you will still be ugly - W.S. Churchill
I should have somewhere a perfect stored bt of Noval 1991 and if I find the bts I will check.
I also have halves from the same source and what i saw was that the filling levels were fine. Overall I was not really impressed with this Port. For me the one of the lesser Novals
regards
WS1
"Sometimes too much to drink is barely enough" Mark Twain