In the loading screen I get a tip saying Breton blood has a 50% resistance to magic but in active powers it says I only have a 25% resistance to magic. What am I missing?

Its a bug. The one in loading screen. %25 is the normal one.

It's likely that when the races were all converted for Skyrim they initially had their Oblivion racials and were later modified, the said loading screen was probably forgotten in the update, in Oblivion Breton had 50% resist magic, in Skyrim it's 25%. If it were 50% it would mean the player could cap magic resistance too easily as Breton, just by going for the Lord's Stone and getting the Agent of Mara bonus.