(B737s) Why is the automatic activation range of the AUTO mode in flight limited to "between idle and 5%"?
The reason to go automatically to ignition below idle N2 is to try to avoid a flameout. If the engine has slowed down to be at or below 5% N2, it is way too slow for combustion to happen - you don’t add fuel to a normal start until about 25% N2, by comparison. So “below idle in flight” almost is a valid characterization of when AUTO will turn on the igniters, it is just that the bottom of that range stops not at zero but at a slightly higher value. Ignition isn’t going to accomplish anything between 0% and 5% N2 anyway. (Probably can’t accomplish much at 6% or 7% or etc either, but those are probably seen only briefly on the way to, or from, near zero, so the engineers picked 5% as their cutoff.)

Note also that some switches are 


GRD-OFF-CONT-FLT


While others are 


GRD-AUTO-CONT-FLT


So, for the switch with an “AUTO” position, there is no other “OFF” position, so there needs to be a cue to the system to turn the igniters off, rather than to let them run continuously on a shut-down motor. And rather than saying simply that OFF happens at 0.0% N2, they’ve specified a range, so that the igniters can be turned off for a shut-down but still slowly turning (windmilling N1) motor.