Investors may be underestimating what the power of compounding can do for Apple (NASDAQ: AAPL ) . If Apple can just sustain its current market share in the coming years, the power of compounding will drive unit volume growth for its iDevices through the roof. According to IDC, Apple is expected to grow its iPhone unit volume by 18.8% a year, roughly in line with the smartphone industry. At that pace, in three years’ time, Apple’s iPhone unit shipments will have increased by 68% since the end of 2012. On the tablet side, it’s expected that Apple will ship 20.9% more iPads per year until 2016, equating to a 77% increase from today’s levels. Are investors missing something so enormous it’s difficult to see? Or could it be that IDC’s estimates are completely off the mark and shouldn’t be trusted?