With linear recording (e.g. old-fashioned analog audio tape) you might well get the best results by playing a tape back on the same deck that it was made on. But DAT recordings are helical and segmented, with a whole servo tracking and alignment system that is strictly a playback-only phenomenon. So DAT playback on one individual deck can have special problems that don't occur on another individual deck, regardless of which machine a particular tape was recorded on. Sometimes, in other words, it is better NOT to use the original recorder for playback. And that occurs most often with miniaturized, single-head decks like the D7 and D8, or the D100 and its quasi-professional counterpart the M1.
Some decks have two playback heads embedded in the drum, some have only one. And as the saying goes, two heads are better than one. They scan alternating segments, and the material read by one head can sometimes get the deck successfully through a patch of read errors from the other head (e.g. if the one head is partly clogged by oxide particles).
The D7 with its miniature head drum definitely uses only one playback head, and the tape has to wrap around it with more of a curvature than with a standard-size head drum. If the one head is clogged or if the tape isn't making good enough contact with the drum, a deck like the D7 will have audible dropouts that another deck, with better tension control and a standard-size, two-head drum, might not.
I still use a D8 occasionally for recording, but I play the tapes back (for transfer to a WAV file) on a full-size deck with a better transport and two playback heads. Also, such things as DAT head cleaning tapes do exist, though they shouldn't be overused.
--best regards