There are some limiting factors that reduce the number of devices we are able to observe. Those can include:
Apps installed on the device and whether the user has opted-in for location services on those apps.
Weak or spotty Cell signal
Venue construction (i.e. cement and structural steel)
Visibility to GPS signals
Length of time the user is physically at a location
Times or dates of collection
Additionally, virtually all major sporting arenas and concert venues back-haul the cellular traffic away from the venue, to avoid network congestion and allow for availability of E911. And, certain locations restrict the use of cellphones (Schools, Theaters, Dispensaries, etc). As we use multiple providers and various types of collection methods, OnSpot typically gets far more penetration of devices than any of our competitors.