ESPs typically report the delivery rate, which only tells you the percentage of email you sent that the mailbox providers accepted for delivery. It does not tell you what percentage of the email you sent went to the inbox or was placed in the spam folder.
Return Path reports on the Inbox Placement Rate (IPR), which measures the percentage of email sent that reaches the inbox.
It is possible for a mailbox provider to accept your email (because you are generally a good sender) which may result in a high delivery rate of 98% or above. However, once the mailbox provider’s spam filter sorts through your sending reputation and starts receiving engagement metrics from your subscribers on that email, then some of the email accepted for delivery may be sent to the spam folder. Depending on the severity of the spam filtering, you could see an IPR as reported by Return Path significantly lower than the delivery rate reported by your ESP.
High delivery rates are important because getting a mailbox provider to accept your email is the first step in reaching the inbox. But, having a high IPR in addition to a high delivery rate is the desired outcome for marketers.