Essence Reaver claims to only grant 20 percent bonus cdr from crit leaving a max 30 perecent possible, however the item dosent actually cap it and you can get full cdr cap off of essence reaver and any 30 percent crit item.
Essence Reaver gives 10% base CDR, if you build at least 20% crit from other items; then Essence Reaver can give a bonus 20% CDR extra - giving you a total of 30% CDR from Essence Reaver. If you are noticing that you have more than 30% CDR; it's more than likely that you either have blue buff (which gives 10% CDR) or are getting CDR from your runes/masteries.
I just tested it with 
and got both 
and 
and ended up with 30% total. You must of had a blue and not noticed.
