Change in pace while walking

[ChrisAtkins]

Two days ago, I went on a tracked walk (Android app) and was doing just over 14minutes/mile for most of the walk until suddenly, the voice cue told me my pace was 13'23"/mile, which surprised me greatly.

Looking at the results from the tracker, as seen here:

The increase in pace is clearly seen about three quarters of the way along the top (pace) trace and was recorded as about one minute/mile, for about a quarter of a mile, something I'm certainly not capable or doing!

My feeling is that this was caused by an error reading the GPS. This is supported by the distance for the walk being about that much longer, too - it's a regular walk for which I know the distance.

I've done the same walk the next two days and not seen the same.

Any thoughts, anyone?

Edit: 20th June. The same thing happened, today (3 days later), on the same route, at the same place on the route, with almost identical characteristics. I'm wondering if it's some kind of RF interference - where it happened is on the seafront/coast path, not far from the RNLI and the Coastwatch buildings.

Moderator Edit: Clarified subject