If you do the sums:
890+45=935MHz – still in band
889.8 + 45MHz – 934.8MHz – yes you are within 915-935MHz – but the -45MHz uplink path is out of band as it is on 889.8MHz.
Likewise in reverse. 915+45 = 960MHz – still in band. 915.2+45 = 960.2MHz – out of band.
Hence to maintain a 45MHz offset you cant use the middle band 915-935MHz for standard GSM. E-GSM can – but then it uses 880-890+925-935MHz in countries that allow that band to be allocated.
Normally the band 915-935MHz is also used by other radio spectrum users, particularly some ISM (Industrial, Scientific or Medical) unlicensed high power transitters, or in some countries Trunked Mobile Radio use it, and in other places, 2-4Mbit/s fixed radio links use that band.