Our SPL modules, including SPL Meter, SPL Pro, SPL Graph, Traffic Light(s) and Dosimeter are all designed to meet the requirements of ANSI S1.4 and ISO 61672. One of the requirements of these specifications is that an overload indicator is required, of a very specific type. 

An overload must be indicated clearly if even a few samples of audio are at the clip point, and the detector must be before any filtering or dynamic processing occurs, such as smoothing or exponential averaging. This can easily be 20-30 dB below the reading of the device in A-weighted Slow response, especially if there is significant low frequency energy. Because of this, you could be looking at 80 dBA Slow values in the 80s and see clipping with the internal iOS device mic for recent devices, which do not support gain ranges.

Even with iTestMic2, which can measure 120 dB typically, 100 dB or lower readings can still result in the overload indicator being shown.

Normally, sporadic overload indications do not materially change the SPL readings, but if you are doing work that requires Class 1 or even Class 2 documentation for legal reasons, the overload should be noted. Try switching to Z or Unweighted, and go to fast or impulse mode to observe what is going on that may be causing the overloads.