The difference is in the protocol that is being used.
The smartphones uses the EAS protocol which uses the Direct Push method so new messages are delivered to the device as soon as they received in the mailbox.
The Exchange protocol connected via Cached Exchange Mode uses a Direct Push/Delayed Pull method. In this case, Outlook is notified directly when the message arrives but Outlook waits 30 seconds before it actually syncs. Any additional server-side changes that occur within these 30 seconds are then directly synched as well.
The Outlook email client is set to use IMAP which could take longer to deliver the mail. Additionally, there are also send/receive settings which can delay it even more. But the most part there is no issue unless you do not get mail for more than an hour.