What you are describing is a point that is often misunderstood and it is good that you bring it up. If you have an MTU of 1500 bytes on the dialler and virtual template and you are running PPPoE, then any and all packets that are 1492 bytes and smaller will be transmitted successfully and any of size larger than 1500 will be fragmented and will pass (if the DF bit is set to 0).
The problem arises when there are packets of sizes 1493 to 1500. In this case, the virtual template and dialler will allow it through without fragmenting it but the PPPoE limitation of 1492 will drop it.
During your testing, most of the packets that were sent were of sizes smaller than 1492 so you didn’t detect any problems. MTU problems will usually show themselves as intermittent connectivity or parts of web pages not showing up, or slow traffic and broken image links. These are very difficult to diagnose.
I hope this has been helpful!