Treadmill accuracy versa lite (stride length?)

[Kmccreary]

Hi -

I've been training for a 10k for the past several months.   I recently ran an 8k race outside and the versa light worked perfectly.   But when I do my work outs after dark,  I run on a treadmill for safety.   Tonight I ran 4.5 miles according to the treadmill but my Fitbit only logged it as 3.6 miles.   Strangely,  the step count and calorie count were correct, so I'm assuming it's not accounting for the increased length of stride while I'm running on the treadmill.  Is there anyway to fix this?   

Note:  I definitely swing my arms while on the treadmill and I run at about 5.5 miles per hour so I'm not holding the bar while I run.   

Thanks in advance. 

[WendyB]

You not covering an exact distance because you are not moving forward.

See this thread by MrMarv
It explains it very well

Treadmill Steps

Another good tread to read on treadmills