Just like every operator promises 95%+ national LTE/5G coverage (they have legal obligations in this regard), but somehow your particular village or town in the lower Alps or in Brittany has shitty data rates for some reason, despite the main street being coloured green on the official covergae map.
Some people in my family got a "zone blanche" subscription from SFR, i.e. LTE with a big antenna, possibly a higher Cat. than regular LTE, because they live too far from any DSL/fiber option.