Hello,
Speed testing consumes large amounts of data - for example, 100MB for testing a 100Mbps line or 0.5GB for testing a 500Mbps line.
If automatic tests are used - than each test consumes that amount of data, as well.
15GB in one month - as an example - could be consumed by a test of a 500Mbps line every 24 hours.
Thanks,
analiti