I believe there are three factors:
(1) Parallax: On FitalyStamp you are tapping directly on the surface of the stamp whereas on regular Fitaly, you are tapping on a glass surface that is 1 millimeter above the actual image. So there is a parallax that results in less precision in regular Fitaly.
(2) Fonts. The fonts for the stamps have been created for easier reading and are printed with a resolution of 2400 dots per inch.
(3) Less display work: On the technical side, Fitalystamp has less work to do with displaying images.
I believe (3) is probably negligible. On the other hand, both (2) and (3) create a feeling of confidence that is essential for speed and this subjective factor probably explains the 10% difference.