I am assuming that 224k records do not match the same Company Name here?
This sounds like a weird mapping error or perhaps a hidden workflow in Marketo or SFDC.
When a field value on the Company/Account object is changed, all lead records associated to the same Company/Account inside of Marketo will have that change made to them as well. The problem we find is that the Change Data Value activity is only logged on the individual lead record where the change was actually made. All other leads associated to the same Company/Account that got the change due to that association will not show the Data Value Change activity at all.
To add to Mike's comment, the reason this does not also make changes in SFDC is because Marketo is not able to edit Account fields.
You're correct about the association between leads that all have the same Account in SFDC being affected, but they only get changed in Marketo.