After trying to prevent air space in carboys after racking. Every time you rack you get more space. Topping it off with a similar wine or mead is usually difficult. And would result in having excess space in other containers.
So last winter I purchased a CO2 bicycle tire inflator with CO2 carriages for around $12. It no longer matters how much air is left at the top, a second or two spray of the CO2 quickly displaces any gasses left in the carboy. The cold CO2 is heavier and pushes other gasses upward. I can even take a gallon and leave the rest to age.
Not sure if this is the reason but my meads have definitely got better