That was not exactly deliberate, but because they failed in its design. The three round burst is pretty standard across weapons of that type. But the FAMAS has an issue with firing standard NATO rounds. So the French in wanting a "FRENCH!" rifle, had to fuck it up and need to use specially made ammo for it. France has been pretty consistent in doing that.
richelieu: 2 main gun mounts, preload
champagne: 1 main gun mount, quick salvo preload
gascogne: 2 main gun mounts, can fire separately
jean bart: 2 main gun mount, first salvo has big pp dmg
Anon is on the right track. Though that extra bullet is supposed to stay chambered in the gun while you reload a magazine in order to prevent having to chamber a new round with the new mag. At least, that's how it's supposed to work in training. People who are good at counting their ammunition can do it well. I am not that good at it.
It may be how it is being trained now, which shows the adaptability of the Military using it. At least the Magazines are not jamming the weapon anymore. Seriously though what system is still using the 25 round magazine? They have had the 30 round mag out for decades.
Most skilled shooters are trained to count bullets so they don't run into situations where they try to shoot and nothing comes out. Even if the chamber doesn't remain open after the last bullet, you would still have to chamber another one when reloading unless if there was one chambered before dropping the mag.
Then they'd have to count both bullets and magazines. I can see how reloading is faster without the charging after changing a magazine... well most only carry 3 magazines anyway.