The way I see it is the Grand Fleet was desperate for organic air support to replace Campania a.s.a.p. so did a rushed conversion with Furious, a more thorough job with Courageous, once they had time to think about what's needed and refined the design with Glorious.
Historically you can see this process with first Furious and Vindictive, then Argus and Finally Eagle and Hermes. (What can we do now? What will do it better? How do we address the problems exposed by the first two steps?)
Sections from Guy Robbins’ ‘The Aircraft Carrier Story 1908-1945’
‘It soon became clear that Jellicoe required more carriers. Williamson visited the CinC on 17th April and gave him the Air Department plans. Jellicoe was primed to ask for Wahine’s conversion and the three new carriers. He also first suggested that the Glorious class carry some seaplanes. Jellicoe wanted a seaplane carrier laid down at once since the only satisfactory fleet carriers were those, which flew machines off and onto the deck. On 22 June, after Jutland, Jellicoe asked for two more carriers because seaplanes had shown their value in a fleet action.’
‘Jellicoe’s main problem – a perennial one - was the accurate reporting of the bearing, course and disposition of the enemy fleet which required experienced observers. He told the Admiralty that the seaplanes flown from Campania’s deck during the exercise had proved of great use, and on 20 July again asked for the Glorious class to be fitted with seaplanes. He did not press for more carriers, but possibly wanted these two ships converted because of their poor gunnery capabilities. ‘
‘Meanwhile in response to Jellicoe’s requests for Holmes’s (Lieutenant Gerard (RNVR)) design to be built and the Glorious class to carry seaplanes, Tudor asked for DNC and DAS to report on Holmes’s design (a design which allowed ships, without stopping, to land aeroplanes aft plus launch and recover large torpedo-seaplanes on a slipway aft). The delay (13 May – 22 June) before the DNC replied was another factor in the lack of action on carrier design in the summer. The problem was that after Jutland the DNC was overworked, assessing battle damage and organising large numbers of repairs.
‘The DAS and DNC agreed that the Glorious class would be delayed by modifications to carry some seaplanes, but they could have been ready for operations in 1917. They were large ships with plenty of room, and if their air arrangements proved inadequate (probably) they could have been converted in turn without depriving the fleet of carriers. No one asked Jellicoe whether the delay was acceptable, nor did he volunteer to sacrifice their gunnery potential (in Furious case very poor) as Beatty did in 1917. Glorious and Courageous were commissioned in October 1916 and changes so late required large and costly (in time and resources) alterations. Furious commissioned later, but Jellicoe did not realise she had only two 18” guns, of little value.’
