This happens when the app has synced but the stored data has not been uploaded to the cloud yet.
This often happens because:
Your phone/tablet connects to your Wave via Bluetooth and then syncs (you will then see "up to date" on the app).
Your phone/tablet will then push this synced data to the cloud (your Dashboard) once it is connected to the internet.
So it is good to check you have functioning Bluetooth and internet connection if you want both your app and dashboard to be up to date.
To check the Last Cloud Sync from the app:
tap on the hamburger menu on the top-left corner of your screen
tap on Devices
tap on My Devices
select your current device
Tap on Sync Info
See Last Cloud Synced