That’s surprising to me if that’s the case. I think the batteries would have to be chemically different and have different (less) capacity when integrated explosives are inside. I’m no chemist though so IDK for sure. Perhaps the power use of pagers is low enough that it’s not noticeable if the batteries have 25% the capacity they should. 

 Apparently the company was setup to do this, so they didn't need to clip capacity of otherwise good pagers but could design them with explosives in mind. Until now, a 4x over-sized battery wouldn't have raised a red flag.