I have looked at 2 different used boards -

RRD 155 l. 258x76 cm. 9.4 kg. Sail range 7-9.4

Bic Techno 148 l. 264 cm x 75 cm. 9.7 kg. Sail range 5.2-8.5

WHY ???

The numbers taken all by themselves, tell you almost nothing. One must also consider the design of the board, bottom shape, width, rocker, and length, but of those four, bottom shape can change everything about that board the most.

My speculation: They are oriented to different sailors. The RRD is aimed at a more experienced sailor who will wind it out with a bigger sail. For that sailor, 7.0 is used in 20-25 knots easy. The Techno is more early intermediate (and I'm guessing more forgiving rails/vee) who doesn't have a 9.5 and will end up sailing in stronger breezes with a small sail because of sketchy to non-existent waterstarting skills. Also, they'll be going to a smaller sail sooner to stay in control as they don't have the skills to downhaul to the max and fly off the fin in way powered conditions.

A more nuanced restatement of what Mastbender said. (And add rail shape to his list.)

Basically it means the bic will be nicer in higher wind .

I guess that the RRD is an X-Ride, 2009. A very good intermediate board, comfortable and reasonably fast, and does work with smaller than 7 metre sails, but, as with all big, widish boards, there are diminishing returns from using small sails in stronger winds. If it still has it's optional fin, you may want to use a smaller one to settle it down in the stronger conditions. We use a 135 twin fin version in Cocos
Both boards are very good for the intermediate sailor, so you won't go wrong with either.

Jez