Walking on rounds with Inspire HR doesn't set pace correctly

[Auja]

I walk on a running track. One round is 400m.I set my Inspire HR to track these rounds á 400m. I start the walking session manually. First round is always right, but then the rounds get longer and longer. By the time I have walked 4 km, my Inspire shows 9 rounds, not 10, and the last round is about 1.25 long = 500m.

My pace varies, but usualy 2+2+2+2+2.

Moderator edit: updated subject for clarity. 

[JohnnyRow]

It is critical to know whether or not you are connected to phone's GPS.  I am guessing not.  If not, distance is tracked by counting steps.  If your stride length is gradually decreasing, that would lead to more steps per track circuit, making fitbit think you are walking longer distance.

How are you setting to track the rounds?  On some models, you can do this by settings Lap distance, but I didn't think that option was there on Inspire series.

By pace 2+2+2+2+2, do you mean 2 minutes per lap?