If you actually read into this, you'd find that phones are quite inaccurate, and so are the cheaper dB meters you can get on eBay and Amazon and the like. There can be variances up to 10dB in some cases, which is quite a lot. It's extremely easy for that slight difference in volume to not be detectable by your phone.
With phones, high sound pressure levels can distort the input, which will completely throw off the measurement. Plus the apps are thrown off by lots of low frequency content:
There was been a growing awareness of sound level meter apps in recent years. But how accurate our sound level meter apps?
www.cirrusresearch.co.uk
Plus, dB isn't the way to compare the final output volume of a playback system. You would want to measure sound pressure level. The REED Instruments R8050 is a decent one.