Or, according to a poster, they appear to have packed the original singles into the box-set, with each probably having ample filler time, ie the FP single of K. 310 (20'00'

, K. 333 (28'50'

, K. 545 (11'58'

disc is under 61 minutes. These three Sonatas were reissued at MP on DG Centenary Collection, and a Schubert Impromptu was added, bringing it up to 72 minutes. Re tempi, you might wanna compare the aforementioned times.