The Dido does not compare favourably to the Atlanta.
Firstly Dido does not have a similar number of guns, Dido has 5 twin turrets, Atlanta has a broadside of 7 twin turrets. which is 4 guns over the Dido
Next Dido has less than half the firerate on the first stage and then only 66% of the firerate during the second stage… On less guns.
Dido has at best equal armour with only the machinery being armoured fully and lacks the US antifragmentation armour which effectively nullifies most small calibre HE shells found on ships below 5.7 with just as weak turrets but probably armoured ammunition.
Dido has less crew, Dido does have better torpedo’s which is a factor but its nowhere near as important as any of the above points. Dido does have better SAP but again it doesn’t offset everything else.
You mention muzzle velocity but you fail to mention that within 10 seconds of being hit by a shell from a broadside Atlanta, you will have received no less than 28 rounds which is more than enough to kill most ships.
You also fail to mention that at range dispersion is a bigger factor so if someone miraculously manages to accurately aim for an ammunition rack then dispersion becomes a factor.
Leander and Arethusa have 6 inch guns with either 3 or 4 turrets and a firerate that is a third of the Atlanta… they also again have roughly equal armour and again better torpedo’s with 2 smaller calibre secondaries.
The comparison is again not favourable.
And again Arethusa and Leander are somewhat on par with things like Karlsruhe which has an extra gun, better torpedo’s and secondary guns which are slightly smaller in calibre but with a vastly higher firerate