Getting into the spirit of things

The Barn Owl box was erected to day in the Widow's Barn. This time, in order to make sure the platform on which it was to sit was level, I had brought my spirit level - no more guesswork, no more doing it by eye. And then we stood back, David and I, and realised that the platform had a distinct tilt, pointing, if my internal compass serves me well, in the general direction of Abaddon. My reaction was to tell David that it was a deliberate tilt, a drainage feature, but he, not fooled for a moment, went back and re-positioned and re-screwed the brackets holding the platform. Then it was straight, parallel to the Earth's surface, no longer Hellbent. Then, box up and woodshavings inserted (one and a half inches: would YOU want to lay eggs on bare boards?), we left.
What I want to know, though, is what went wrong with the spirit level. Has global warming done something to the bubble, giving it a cant to compensate for the reduction of the Greenland ice-shelf? Hell, if you can't trust your bubble, you could end up living your whole life on the slant.