Why do I get a security alert AFTER installing 4.6.14?
There is an open issue on this: CRM-18156.

This logic is implemented in CRM_Utils_Version:: checkBranchForNewVersion() -


check as implemented in v4.6
check as implemented in master / v4.7


Update: Now fixed, cause was identified as data in the pingback / version check feed on CiviCRM.org. See JIRA issue CRM-18156 for details.