Questions about aria-rowcount, etc.

I'm trying to figure out the correct user agent implementation of the new
ARIA 1.1 row and column attributes.

Let's suppose we have an HTML table element with 3 actual rows.

Clearly if the author adds aria-rowcount with any number >= 3, that's the
number of rows the user agent should expose via native accessibility APIs.

If the number of rows is unknown and the author sets aria-rowcount="-1", my
interpretation is that the user agent should not expose the number of rows
to native accessibility APIs - perhaps by returning an error when queried
for that value.

What if the author tries to set aria-rowcount=2, when there are actually 3
rows? I'm not totally clear on what the user agent is to do in that case.

The spec implies the author shouldn't do this, but it's not clear if the
user agent should pass on the value of 2, or ignore it and return 3 because
ARIA isn't allowed to override the native semantics.

Thanks,
- Dominic