If you use PowerShell to list members of dynamic distribution Groups in Exchange 2013 you would probably see more entries than expected. The PowerShell commands

$dgr=Get-DynamicDistributionGroup “GroupNAme”
Get-Recipient -RecipientPreviewFilter $dgr.RecipientFilter

does not take in to consideration OU filtering. If you look at $dgr.RecipientFiler it only contains the other attributes.

I do not know if this is a bug or by design.