The fluid six had the three groups of two aircraft at different heights - which meant it would be harder to bounce the formation. You also had two extra pairs of eyes looking for enemy fighters - the more the merrier !
From Wiki ;
One tactical innovation adopted by 2nd TAF Spitfires was the "Fluid Six"' formation, which had been developed through combat experience in Europe and North Africa. The first use of the tactic dated back to at least November 1941. It is known that No. 112 Squadron RAF used this in the North African campaign.[100] This formation "was considered the best fighter formation of the war".[101] It abandoned the leader-wingman combination that had existed before. Instead, it was based on three pairs of Spitfires which could provide mutual cover and support: the pairs were 'stacked' in altitude so that the pair (e.g.: 5 6) flying up-sun, and covering the tails of the leaders (1 2), flew higher, while the other pair (e.g.: 3 4) flew lower. Any attacking aircraft could be sandwiched between two pairs of Spitfires, no matter the direction or altitude of the attack. Another advantage of this formation was that when operating at squadron strength a flight commander was able to lead six aircraft of his own flight, "whereas, with formations of four there would more likely be one formation from each flight with the third consisting of aircraft from another flight.