Yes, that is common though. You have a variety of measurements, from a variety of technologies, from a variety of distances, and from a variety of sources. Given the damage a quake and tsunami can cause, especially the early measurements are estimates that later get corrected in light of new information. In Japan for example, it is very much not uncommon that early tsunami warnings are later cancelled. Yes, false alarms are bad, but the example I usually use in terms of how much time can matter is the 1983 Sea of Japan quake [1], where the tsunami hit in 12 minutes after the quake.

[1]: https://en.wikipedia.org/wiki/1983_Sea_of_Japan_earthquake