Claiming land: Your diamond mine may be unsafe!

When I started on the server, I claimed a small section of land, then as I progressed, I claimed a bit more next to it, then another bit. After a while, I had difficulty with trying to expand because of overlapping claims, so I removed ALL my claims, and claimed one area that encompassed them all...

Anyway, I have just taken a stroll down the stairs in my mine, trying to work out how far out the claimed area extends - does it cover all my mine below my house? The answer is no, it doesn't, but not because the claim borders don't go out that far, but because EVERYTHING below y=32 is unclaimed. To illustrate, 2 screenshots taken from exactly the same position:

Looking at y=32:

Looking at y=31:


So, everything below this point is unclaimed. I assumed that a claim covered "sky to bedrock", but it doesn't. Is this intentional? Should I just make another claim at the bottom of my mine, and it won't overlap?

This is odd...
I dug out a little area on the side of the stairs here, and checked - the problem still existed.  I told 19Plaz_CC about it, then placed 2 signs:

I checked again, and only down to y=27 was claimed. I placed another sign (blank one is fine) at y=26, and it extended down again. I then went down to y=6 to find it was still unclaimed and placed another sign.

I haven't yet checked it all, but the claim system is definitely glitchy - other players may be able to get to stuff below your claim - be careful, and check it with a stick...