Because there is no current flowing across it, there is no internal resistance which is the origin of the difference between voltage (p.d.) and e.m.f.
I’ve never been asked this before, so never thought about it (which makes it an excellent question) but I think this is it.
There is only a difference between e.m.f. and the voltage reading when a current flows because there is a potential dropped across the internal resistance (the ‘lost’ volts of a cell.) The size of the difference is equal to the internal resistance, r multiplied by the current, I.