Why do I need to take immediate action when my mailbox size is 4,208 MB when the quota is at 5GB?

Why do I need to take immediate action when my mailbox size is 4,208 MB when the quota is at 5GB?

I just received the following email from Zoho Mail:
Your mailbox size is nearing the quota alloted to you.

Your current MailBox size : 4,208 MB

We request you to take immediate steps to clean up your mailbox and create free space for your email account to function without any issues.



I am a free user and from the FAQ I understand that my mailbox quota should be 5GB. How do the 4,208MB in this email relate to 5GB? Isn't 5GB = 5*1024MB, which would mean that I have plenty of room before running out of space?