Its simple when the OEM gets approved by a Mobile Operator at least for the USA ones the MOs are negotiating an update fee that they pay OEMs for that very purp

Its simple when the OEM gets approved by a Mobile Operator at least for the USA ones the MOs are negotiating an update fee that they pay OEMs for that very purpose.. So its probably set up as 3 Major version updates as Google/OHA members want that kept at every 6 months..hence the 18 month time limit..