Frame sizing: "medium" vs. inches vs. centimeters
Essentially, bicycle frame sizing is inconsistent and possibly insane. Try the bikes out. Don't rely on a single number for the size.

Theoretically the size given is the length of the seat tube, but there's a lot of inconsistency in how that's measured.

Usually a frame size given in inches is a mountain bike frame and a frame size given in centimeters is a road bike. A mountain bike frame will have a higher bottom bracket in order to clear obstacles better, and may also have a lower seat tube (to lessen the odds of your crotch colliding with it when dealing with rough terrain), so you can't simply convert one number to the other. If you look on that chart and compare the "hybrid" sizes to the "road" and "mountain" sizes you'll see that hybrid cm is close to road cm and hybrid inches is close to mountain inches.

Even within a single basic bike type there can be a fair amount of difference in the bottom bracket height, which directly affects standover height.

Basically, mountain bikes and road bikes have two different sizings and you need to find out the appropriate size for you in each. Hybrids seem to sometimes be sized like road bikes or like mountain bikes, but the frame geometry probably doesn't really quite match up to either style of bike and they may report the sizing as what matches the other style or as the actual measurement. There's a lot of variation in hybrid geometries because there's a lot of different sub-types of hybrid intended for different kinds of riders.

Because the manufacturers aren't consistent, you really need to try the bikes out to find the right size bike. Two bikes of the same basic type may be the exact same height but have different top-tube lengths and therefore fit you differently. If you're an experienced rider you might be able to work out what you need to know for a bike given the full geometry information that many manufacturers publish, but the single number size will never be enough.

See also: http://www.sheldonbrown.com/frame-sizing.html