Question about height of column in my airlocks

[BernardSmith]

I am curious about why after some months aging (six months or more in some cases) and after I have stabilized all and degassed many using a vacuum pump, a number of carboys all of which have simple one piece S bend airlocks indicate pressure coming from inside the bottles. This does not appear in every airlock and so I assume that suggests that the cause has nothing to do with ambient air pressure (assuming that all the airlocks have about the same amount of water in them).
Do you typically find that your air locks have very different heights of liquid in the two halves of the S bend? What is the cause of this? As always thanks for reading this post and for your responses which I always enjoy and find interesting and useful

 

[DirtyDawg10]

Could be barometric pressure or thermal expansion and/or contraction. Mine move around a bit but I do find that most of the time the liquid in the airlock is being pushed toward the outside.

 

[Dugger]

I think the differences can also be attributed to the columns starting out at different levels. Many times I will insert the bung with a filled airlock and the column will be pushed toward the outside simply from the insertion process. This can cause a variance from carboy to carboy at the very outset and this may continue during the aging.

 

[jswordy]

Evaporation of alcohol. It does not take much to push that lil column of water up.