Originally Posted by Server

It's mostly due to cached inventory. Various buckets are opened and closed to the distribution system.

The airports being searched might have had a change in frequency of searches. Usually if it changes more often it will not cache, but if does not change prices often or dramatically your city will get old inventory. Thus will not synchronize with the GDS often.

Not a big deal since before Google flights existed, ITA required you to book it off-site anyway.