Were you adding the 5.2 to the mash or just to the water with no grains? I assume, just the water. If your water has a naturally high buffering capacity, it will take more 5.2 to counteract that. When you add the grain to the water, the organic acids, etc. in the grains bring the pH down (or up depending on your water). Depending on the buffering capacity of your water there may or may not be sufficient compounds in the malt to obtain the proper mashing pH.
If you want a chemistry analogy, here's one. Making up number here.
Say your water has the buffering equivalant (buf. eq.) of a 50 mM solution, pH 8. Let's say 3 gal. worth
Say your malt when dissolved in 3 gal of water has a buf. eq. of a 100 mM solution, pH 5.2.
In this case when mixed, your mash pH will be 5.2, no matter the original pH of the water (you've exceeded the buffering capacity of the water - 100mM > 50 mM, that is all that matters)
Now lets reverse it, 100 mM for the water and 50 mM for the grains. In this case the pH or your mash will be 8*. Now you have to add enough buffer 5.2 such that the concentration of the (malt buffer ions + the 5.2) is greater than 100 mM. Then your pH will drop to the proper range. If you don't add the malt, it will take a lot more 5.2, as you don't have the help of the buffering compounds in the malt. This is why you should always add the grains, mix, and then check the pH.
One can add either acid or buffer to lower the pH. If only acid is used, you will lower the pH of the mash but also lower it's ability to resist further pH changes (like while sparging). You have "used up" some of the buffering capacity of the mash. By adding the 5.2, you will lower the pH and increase the buffering capacity of the mash (or at least not lower it as much as if an acid is used)(lactic acid and phosphoric acid do have some buffering capacity, but the pH range is lower than that of the mix of phosphate salts in 5.2).
Acids and Bases will raise or lower pH, and the function of buffers is to resist pH changes, but you can use buffers to change pH too. If one tries to tirate a strong acid with a strong base it is very hard to hit a particular pH somewhere in between the two. It will go from one extreme to the other very quickly, often with one drop of either. Now if one uses weak acids and bases, the change is much more gradual (in the pH range of the buffer). It will take a lot more to make the pH move much. Now, once you exceed the buffering capacity, these too will quickly go to the extremes
*This is what would happens when you mix standard buffers in the lab. In the real world it will probably be less than 8 as the natural buffers in water aren't real good at it - ie. some buffers resist pH change better than others at an equal concentration (phosphate is better than carbonate).