I think it is because Bubbler is not satellite based and can have long periods of interruption. The IBA requires position reports every 10 minutes. There are still many places in the US that do not have cell phone coverage.
Spot can loose signal as well. While satellite is not as susceptible as a cell based system, there are some areas in the West that can interrupt Spot tracking.
A friend and I were riding in Utah and he lost tracking through some canyon country. GPS went south as well. Good thing we had some maps!
And - a GPS can be "rigged" to indicate incredible top speeds.