As described, your computer may be configured to use POP3 mail - while your iPad is using IMAP - these being two completely different connection protocols. IMAP is intended to keep all your devices view of email synchronised between all your devices. By contrast, POP3 (an older protocol) is considerably more difficult to reliably configure for multiple devices.
When using IMAP, your device(s) will download a copy of your email messages when fetched from your Mail providers email server. The Mail server will keep the original email messages until such time as they are proactively deleted from either your iPad or iPhone.
If instead you are using POP3, email clients can be configured to (a) download and leave a copy on the server - or (b) download and delete from the server. As such, email messages may be deleted from the Mail Server as soon as they are downloaded to your device(s). You may see synchronisation issues, or find that you only see new mail on one or other of your devices.
If supported by your Mail provider, it is also possible to configure a mixed environment where some devices are using IMAP and others POP3. Doing so requires considerable care - as misconfiguration can lead to unexpected behaviour. IMAP is the more modern Mail protocol that, for most domestic Users, is the preferred option.
You will need to check the configuration of your computer. Assuming that the computer is using POP3 as diagnosed, you’ll need to reconfigure your computer to use IMAP for email.
Very often, if you are uncertain as to how to make the necessary changes, the easiest fix is to delete your email account from the computer - and then re-add the account, being certain to select IMAP during the process.