From the u-blox MAX 7 chip data sheet (Max 7 data sheet) is possible to conclude that there is a option to improve the accuracy of the GPS by activating the SBAS feature.
This option is enabled when the Power Save Mode (PSM) is not activated.
Thus, when the polling interval is smaller than 20 seconds.
When the PSM is enabled then the SBAS option is disabled for battery consumption optimisation purposes.
However, If you wish to have a polling interval bigger than just 20 seconds and have the SBAS feature activated you can create your own firmware from our SDK page and have the PSM disabled. Consequently, the SBAS option will be enabled automatically.