Depending on your messaging provider, the cart HTML generated for an email address is either loaded into a profile field, for example cart_html, on the messaging provider itself, or it is requested at send time from a URL and merged into the email's body.
This content is created on our system each time a a trigger tries to send an email, or when you use the test send functionality. At this point the HTML is created and either loaded into the messaging provider, or cached on our servers to be retrieved when the messaging provider generates the email.
Either way, it's not updated until a trigger or test send cause it to be regenerated. This means that, if you do a test send from your messaging provider rather than our system, it merges in the existing HTML and you won't see any changes that have been made to the cart in the meantime.
To avoid this, just trigger the email through our system to ensure it's updated each time.