perfectly pure RO water has a pH of 7 and no buffering capacity. BUt a slight diffusion of CO2 from the air will easily lower the pH. But since there is no buffering capacity, if you mix RO with any buffered water, then the system will reach a pH between the two, but close to the buffered water pH.
..now, as to why the pH was above either of the two when you mixed them, that points to either test failure, or user error on the testing....or, if your tank had a lot of disolved CO2, the turbulent mixing of weakly buffered total water, could have outgassed CO2 causing a raise in pH.
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