Hi rod...
Fly fishing is full of contradiction, the rules are generalisations, they date back to the dark ages and dont really fit any more
the up eye for dry and down for wets is a relic of how flies used to be tied onto gut using a turle knot to make them sit properly on the water, with thin modern mono filaments this is no longer applicable
cock hackles are stiffer than hen, they tend to distribute weight better and aid floatation of flies, whilst softer hen hackles work better on wets as they give more movement.
finally wets are considered suggestive flies, they give the impression of being something good to eat without being imitative, nymphs are the opposite, imitating the larval stages of macroinvertabrates
there are many methods of dishing both wets and nymphs, some are similar (across and down for example may be employed with both wets and nymphs) whilst czech and french style ae exclusivly nymphing methods.. it will become clear with time .