The most common cause for this is that the record in SFDC is not the same record as the one in Marketo. This can happen if there are duplicate records in Salesforce with the same email address.
To check if the record in Salesforce matches the record in Marketo, compare the SFDC ID. On the Marketo side, this can be found in a field on the Lead Database record. The SFDC ID for a record in Salesforce can be obtained from the URL. It is generally the the 15 alphanumerical characters near the end of the URL (e.g. 012300000012BYN) - here is an article in Salesforce for more information: https://help.salesforce.com/articleView?id=000004383&type=1