OpenXR Toolkit counts the number of frames submitted by the game (xrEndFrame). There is very little room for this to be incorrect (read: there is no room for this to be incorrect).
Most likely, the reason you might see different numbers with dev mode is because it might use a different averaging method or period. OpenXR Toolkit uses a “sliding window” of 1s (meaning: the number you see is the number of frames counted in the last 1s, there is no averaging per sei, it keeps a timestamp for all frames submitted, and removes frames from the count when that timestamp is older than 1s).