This happens only under one scenario for Smartlead, right now (dated 24th March)
If you have a campaign with 4 sequences and 1000 leads.
The campaigns kicked off and well underway, after a few days you add a new sequence 5.
The new sequence 5 is setup to be delayed from sequence 4 by 3 days.
In this case, all the leads that have already received sequence 4 will immediately enter the cycle for receiving sequence 5 mails.
However any lead that is in sequence 3, 2, 1 or has yet to start will adhere to the 3 day gap between sequence 4 and sequence 5 as they are yet to "reach" sequence 5.