1. Small letter "m" stands for milli- prefix:
https://en.wikipedia.org/wiki/Milli-You can't use less than one bit so millibits are nonsense.
If you want to use M for mega, you should know that software in most cases shows binary units (mebibytes, 1 MiB=1'048'576 bytes, though traditionally MB acronym is in use), while ISP bandwidth is counted in decimal units (megabits per second, 1 Mbps = 1'000'000 bits per second) .
2. FDM counts only data which can be saved as a file, that is - in bytes. You cannot save one bit file. Total number of bits depends on transmission protocols and is available only from hardware drivers level. See explanations in older topics:
viewtopic.php?f=1&t=17626&p=44148#p44148viewtopic.php?f=1&t=16868&p=37965#p37965viewtopic.php?f=1&t=11205&p=32472#p32472In most cases you can round 1 MB/s download speed to 10 Mbps bandwidth. Ethernet UTP/STP cable transmission has smaller overhead, Wi-Fi connection has larger overhead.
3. Don't trust speed testers. They recalculate download speed to bandwidth as I have mentioned in p.2. What is more, they show only approximate data, just to have any orientation about current bandwidth available.