Is it normal that the mid & high are very close for a 18650 led flashlight?
Depends on the light.
Some write it up as a negative thing in a review.
and of course it also depnds on the total aount of groups a light have
low med high makes spacing more important between the modes then
1 ulra low 2 low 3 med 4 brighter med 5 high 6 turbo
The idea is often that the high setting can not be used continuously because of overheating, and the next level, even though it is close, is the maximum level that can be used continuously.
But I don’t like it, a decent mode spacing is more important to me.
I think a lot of it has to do with driver designer’s failing to take the way our eyes see light into account.
As we have to double the amount of light to make a noticeable difference in what we perceive:
(Stealing one of Match’s output graphs from Emitter Test Results pt4: XML2, XP-E2, XP-G2, and Nichia219! (along with sinkpad tests))
Say, for instance, a designer wants a three-mode High-Mid-Low, so he builds a driver that steps in thirds from 3000mA to 2000mA to 1000mA. At 3000ma, the emitters is giving about 1k lumens. Step down to 2000mA, is using only 2/3 of the power, but is still putting out about 800 lumens. Your eyes would see this as being significantly close to the same output as the High mode. On the Low mode, the light would be operating closer to what your eyes would see as the Medium.
Based on this, a better possibility would be a mode group as:
High 3000mA, Mid 1200mA, Low 400mA
What keltex said is a good tech answer 
Additionally, a poor cell with a high internal resistance will not give max output in the highest mode so will seem very similar to medium output. What flashlight and what cell are you using Ben?
I see it most commonly in AA lights with a protected 14500 installed instead(I’m leery of imr’s in this usage) med gets pushed right up against high with little separation.