Short answer:
What you’re describing usually comes down to how the Find My network works, rather than a “real-time GPS” difference between you and your wife’s iPhones. AirTags don’t have built-in cellular service — instead, their location is updated whenever they come into Bluetooth range of any Apple device on the Find My network. If your wife’s phone is getting fresh data but yours isn’t, it’s likely that her phone (or another nearby Apple device near her) has recently picked up the tag’s signal and uploaded it, while your own Find My app is just showing the last location your phone knew about.
A bit longer answer:
When you share an AirTag, each person’s iCloud account maintains its own “last known location” record for that AirTag. That record isn’t a single shared database entry — it’s separate per account, and each account’s copy only updates when that account’s device or another Find My network device uploads location data that Apple’s servers tie to that account’s access rights. So, if your wife’s iPhone happens to be in an area where another stranger’s iPhone or iPad detects the AirTag, her iCloud record will get a new timestamp and location immediately. Your iCloud record might lag until one of your devices requests and pulls down that new record, which can be delayed if Find My on your phone isn’t actively refreshing or hasn’t had reason to update yet.
Also, the Find My network prioritizes updates from devices that have recently been in proximity to the AirTag. If your wife’s phone was the last one physically near that AirTag (even days ago), the network sometimes routes fresh updates to her account’s record first. It’s not a perfect “who’s closer” system — it’s more of a “whose account has the freshest handshake with the AirTag” system. That’s why she can be miles away and still see the new location before you do, even if you’re right down the street.
One potential option, if you want both of you to see nearly simultaneous updates, you can “prime” your account by physically being near each AirTag at least once a day so your devices stay in the AirTag’s recent contact list on Apple’s servers.