Of course, there are several possible factors, that modify the simple, ideal behaviour. One point is friction, that involves an motor idle current, which is also speed dependant. You may want to measure it by operating your machine with no load. If the machine involves a gear, there may be additional torque and speed dependant losses.
Generally, if the ideal equation doesn't fit, you have to measure the real behaviour and try to define a corrective term. But cause there's no better ideal equation, this is pure empirical, although it may lead to simple linear or quadratic expressions.
As an additional remark, during speed up or slow down of the coil, there's a considerable effect of inertia.