In that case I would suspect that Google throttles the API in use by Talkatone, and either bypasses or limits to a higher number the transactions through the site. The interesting question will be what is the period on that limit... 24 hours, a week, a month...
I do know that Google's SMS APIs are non-public and they strongly discourage their use by apps like Talkatone, but that to date they've been ok with it.
Have you installed Google Voice for Android and tried using that to text? It might be a workaround/longer term solution.
I suppose it's also worth asking whether you're using Google Voice as your primary number, or simply using it to make outbound calls when on wifi.