The manufacturers spec for a rear derailleur sheet will list 2 different numbers. One is the max number of teeth on the cassette and the other is the total number of teeth that the derailleur will take. In your case 25-12 for 13 in the rear plus 48-39 for 9 in the front. 13 + 9 gives you a total tooth count of 22. This should be within the specs for a short cage rear derailleur. The spec sheet is conservative.