It may be different for every tank but my 60 gal. Eaton is set up horizontal tank style and I have a 5 gallon pancake that does this same thing I've noticed. I normally leave it charged with the switch off and the ball valve at the output closed when not using. But at least for my set-up, a short opening of the bottom drain is not the most reliable way to get all the water out. If I have ignored it long enough to get a little moisture built up and then open the drain all the way I get a little water, then up to a minute of just air, then another blast of water. I think the rushing air overpowers the water and pushes it out of the way up to a certain air speed and then once the pressure is bled down some the water finds the valve. Unless you open that thing all the way up every so often and bleed it all the way down, there could be water in there that you don't realize.
Anyhow, so I crack it open a short blast daily when using and do a full drain more like weekly.
Who am I kidding?...I crack it open somewhere between 3 times a day and once every 4 days and I do a full drain at somewhere between 3 and 17 day intervals.