You know, from the pics, it's upright. It also has an "inner" cap, meaning the cap threads down into the inside of the end.
This is not conducive to shedding water. Rain will sit on the flat top, laying on those threads. If there is a way in, water will find it.
Also, if it's as close to the surface as it appears, and the sun hits that area, the top will warm up in the sun, and cool at night.
A ring always expands more than a plug.
The air at the top will add slight pressure, if it can escape, the cooling at night will cause a vacuum, sucking air back in.
Or, some rainwater that was on top.

At least that's a theory.

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