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Does anyone know what the difference is between wet and dry boiling point in brake fluid?

since brake fluid is hydroscopic it absorbs water. in less than 24 months in a system the fluid will absorb enough water to get to its "wet" point through the air. Even if you never remove your master cylinder cap. so you'll notice the wet boiling point is signifigantly lower.
This is why its reccomended to change your brake fluid every 2 years. if you never overheat your brakes not such a big deal. if you do overheat them then they start boiling and the water in the system turns to steam and you lose your pedal.
The other bad thing about wet brake fluid is it can cause rust inside your master/slave/calipers...

The wet boiling point is determined with fluid contaminated by 3% water (by volume).

Minimum boiling points to meet current specifications
Polyglycol-either based fluids:
            DRY     WET
DOT 3   401ºF   284º F
DOT 4   446º F  311º F
DOT 5.1 518º F  375º F

Silicon based fluids:
DOT 5  500º F  356º F
...silicon fluid is not hydroscopic and water won't actually mix with it, but it'll still get into the system - and boils at 212°F.