Here's my take on it for what it is worth:
Soil is (hopefully) a living, dynamic system whose composition is anything by homogeneous. The pH of one scoop of soil from one part of my garden is likely to have a slightly different pH than that of a scoop from another part of the garden. Similarly, other metrics I might collect on the soil will likely differ as well, though probably not significantly unless I'm purposefully trying to push the pH in one bed toward the more acidic side of the scale or toward the base side of the scale. Variation, probably slight, is to be expected though it is possible that the testing conducted might not indicate much of a difference.
The thing is that anytime you do a pH analysis (or any analysis for that matter) of the soil, you are taking a representative sampling of the overall soil. You're not checking the entire bed. The idea is that the sampling will provide you with adequate information to determine what, if any, changes need to be made to effect a desired change in the soil composition. So, if I am concerned about a pasture's health, I'll collected samples from several sites in the pasture and send it off to the Virginia Ag extension folks and they'll test those samples and provide guidance based on their findings. This is based only off the samples collected but the point is that those samples will provide a pretty clear picture of the whole.
My advice is to mix your soil well and then test and trust that the testing will provide you with accurate information for the bed as a whole. The soil isn't homogeneous but it doesn't need to be either. As long as you are thorough in your mixing of those components, any sampling you do will be good enough for the entire bed. If you doubt that, then take several samples from various positions and average.
That's what I do, when I concern myself with soil pH. With blueberries, which thrive in the more acidic range, pH is important but getting the components mixed will yield you samples that are accurate enough.
One more thing to remember - everything that you put in the garden may vary itself in its pH a bit - peat is a natural product and it will vary a bit. The good thing is that plants are resilient and as long as you are in the pH range, everything will work out great.
