I noticed while building an item set that 
only rewards +4 mana instead of +8 mana like 
. Not sure if this is a bug or intentional, but it seems to be unfair and useless to build to it until 
is fully or near fully charged.
Pretty much intentional. Manamune can proc on any spellcast or basic attack, where as Archangel's must be procced on spellcast. That's why it's so much lower (plus maybe the cheaper price on it for a quicker upgrade as well).
Tear of the Goddess gives +4 Mana up to 2 times per 8 seconds on mana and spell casts; +1 mana per 8 seconds~ Manamune grants the same as Tear but basic attacks also stack the mana as well as mana consumed and spell casts~ Archangel grants +8 Mana instead of +4 Mana up to 2 times per 8 seconds, everything else is the same as Tear~
These are the intended effects of the three items of what i listed
