You should be getting 1GB of high-speed data (~12.3 Mbps). Once you have used the 1GB you will have an additional 1GB of data at a slower speed (~3.5 Mbps). The total being 2GB of data as advertised.
I know this is an older feature but I do not remember it behaving this way. Typically a data feature would provide you with the said amount at the highest-speeds available in your area and once that amount is hit, it is still unlimited but at a slower speed. I do not remember a feature that had 2 tiers. If you have a moment, please contact us so our care team can look at your account and your specific data feature to give you the most accurate information.