IMAP storage issue

One of our customer has configured his Exchange account through IMAP. 
Now is the problem that his imap account on the pc says that there is 9.92 GB data , but in our server this is 9.26 GB 
So IMAP says to have more then our server, do you know how this is posssible and why it does not sync with the exchange server? 
Mudesira MunirEnterprise Support & ServicesAsked:
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JohnBusiness Consultant (Owner)Commented:
That is pretty close in size. Could be an older email in IMAP that downloaded to the PC and is no longer syncing, and could be an attachment in such older email not syncing.

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Alexei KuznetsovMicrosoft Outlook MVPCommented:
It's OK. Local IMAP cache (OST file) contains more data (metadata, format specifics, etc.)
Shaun VermaakTechnical SpecialistCommented:
The OST database and IMAP mailbox are different formats and save things differently
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Alexei KuznetsovMicrosoft Outlook MVPCommented:
Nope, Shaun. Outlook uses local OST cache for IMAP mailboxes.
Shaun VermaakTechnical SpecialistCommented:
If you prefer, the online mailbox and the OST are different formats
JohnBusiness Consultant (Owner)Commented:
@Mudesira Munir  - Thank you and I was happy to help.
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