You need to be certain about what these statistics are measuring, when they are measuring it, and over what time period.
The online speedtests are providing a snapshot at a particular point in time, but it is not clear what information your router is providing - is it current peak? daily max? daily average? average since last reboot?
If it is also user-initiated, is it measuring data transfer rate in the same way as the online tests? And to / from where?
The external tests agree so much more closely that they are probably more representative of the real-world situation than what your router reports.
Without more detail about what your router test is actually reporting, it isn't possible to do more than conjecture.
SD