Why a manager would say they are not hiring even though a job posting is recently updated.?

A lot of those websites are actually using postings to do salary surveys, see what people in such positions are earning elsewhere. Sometimes they are building up a referral list for an anticipated opening which is not yet open. Just because a "job" is advertised does not necessarily mean there is a current opening.

In a large corporation, local managers quite likely don't know what the corp has posted in job wanted ads. If you just randomly showed up at some store that's exactly what you should have expected. Probably a good idea to submit your resume thru the online ad.

If a company says it is hiring, then they are immediately open to demands for interviews and lawsuits for discrimination because they will not interview someone. Been there and done that as an employer.

The are collecting resumes for possible future openings.

Probably they have just hired someone, or they have a candidate that they want to schedule a 2nd interview with..

Many firms do this. They encourage applications even though they are not actively recruiting, so that if they suddenly need someone they have plenty of up-to-date applications to choose from, or if they are approached by an exceptional candidate they can 'create' a job for them.