To begin with, few engines hold a perfectly steady RPM. There is always some variation, small but there. Digital tachs respond to this variation, and some probably add a bit of their. In all cases that I have seen, the displayed RPM fluctuates, more in some cases than others, but it is almost always there. It is often hard to com eup with even an average reading.
On the other hand, the analog tach, those with a mechanical needle, do not respond to the engine's variations, and being relatively slower to show a reading, actually average out the variations and display a valid value.
I am referring to "engines" - glow and gas. I have no experience with electric "motors" but with many I see advertised, I would imagine some differences in quality is to be found. The same applies to speed controls. If there is a variation in the props RPM, the digital tach will increase the error displayed. The analog tach should provide a more useful average information.