The data myPWS requests from the WU server to create charts are aggregated data of the last 5 minutes. That means that WU collects data from your PWS, aggregates that data, stores it on their server and delivers it per request to any third party app, just like myPWS.

How the data is aggregated and how quickly the aggregation and processing of data is done by the WU servers is something we do not know. myPWS depends on the data the WU server delivers and myPWS can only display the data delivered by WU. That means that if you think that there's something wrong with the data displayed, you'll need to contact WU support as only they will be able to explain the data delivered for your station.

The WU data servers are known to be pretty unreliable sometimes, unfortunately. If myPWS displays outdated data for your PWS, then the WU data servers delivered the outdated data for some reason. Why this happens sometimes: only the WU staff might now.

Before contacting WU you should make sure that your PWS actually delivers data to WU and that it does that reliably. Such as myPWS can only work with the data delivered by WU, WU can only work with the data delivered by your PWS. Make absolutely sure that your weather station is set to the correct time zone. An incorrect time zone is often the cause of missing or implausible data.

Wunderground Support: https://www.wunderground.com/about/contact-us