That variation in pH is totally statistically insignficant. You could play around with various hypotheses, but my guess is the water is moving around and reacting to the air, out-gassing a little in the local area you're testing (as in aeration), re-mixing a little unevenly, etc.

Sometimes we over-measure and over-organize. One company that made sort engines as a gag produced a calendar that was completely alphabetical for days of the week and months of the year...
Friday, Monday, Saturday, Sunday, Thursday, Tuesday, Wednesday...
April,August, December, February, January, July, June, March, May, November, October, September
Kinda useless but very funny!