It also depends upon whether the ships static system shows true
static or not. If you read through Richard Johnson's tests you'll see
that many gliders have static systems with significant errors built in
to them. If there is more than one static source the manual will
usually specify which one to connect to as that is the one which was
used for certification testing and the placard speeds will take any
error into account.

I would reconnect everything the way it's supposed to be and then get
the pitot-static system checked out with a test set.

But wouldn't a leak in the system cause the airspeed to read lower?


That depends on whether his cabin pressure is higher or lower than
static.