OK, well cancel my last useless guess. Here's a new one.
When you select font sizes Photoshop refers to a selection you made in Preferences > Units and Rulers. If the resolution of your image is 72 ppi, points and pixels are equal. If your resolution is higher one point is equal proportionally more pixels. So, you can select points in preferences if you want the type to scale text according to image resolution but use pixels when you want the type to map the text to an exact number of pixels. You can also use millimeters instead of points.
Then there is the issue that type is supposed to be measured from the top of the ascenders (b, h, d, etc.) to the bottom of the descenders (g, p, q, etc.) However type designers don't necessarily follow those conventions so even commonly used fonts will vary greatly. The only solution is experimentation. I've long ago stopped thinking that point size means anything more than a relative number in Photoshop.
Does that help? \:]