UE4.11 preview 1 - Rendering Test

https://farm2.staticflickr.com/1455/24142068909_cf70af5014_k.jpg
bathroom5 by simfactor, on Flickr

Out of curiosity I built the lighting of the same scene (picture above, without changing any parameters at all) three times. I was under assumption that the build time would be the same through out the test. However, the result is really really unexpected. Here it is:

First build time : 14:42 minutes
Second build time : 15:28 minutes
Third build time : 22:18 minutes

This test was conducted using UE4.11 preview 3 and no parameters/values were changed between the test.

So the first question that came up to my mind was: Is this a bug?