Matching Rules Criteria returning too many rows

Hello,

I'm helping our 3rd party integrator configure a duplicate rule validation on our Account Data. The scenario is this:
- On our testing environment, we have around 150 Accounts only.
- We implemented a matching rule that has just 1 single criteria, a specific field in the Account typically containg an ID Card or Passport Number.
- Matching CriteriaAccount: IdentityEXACTMatchBlank = FALSE
- When we try to activate account, it fails and we get the following e-mail:

You tried to activate the matching rule Identity No. Matching Rule for identifying duplicate records. However, we can't activate the rule because it defines an unusually large number of records as possible duplicates. Specifically, take a closer look at these fields:
Identification No.
Edit Identity No. Matching Rule to include a field that contains more unique values, and then try activating the rule again.

Now I've examined the data, and out of the 150 or so Accounts, only 4 or 5 have such duplicates, so I cannot understand why the above is happening. Got any suggestions?

Kind Regards,
David Captur