If you build a profile form and expose to the frontend as a search form, built-in fields have an implicit wildcard before/after included. But custom fields do not. This is confusing for the end user.
For example, if I put "ria" in the firstname field (a built-in field), it will return everyone with the firstname Brian.
If I put "new" in a custom field, it will return records that ONLY have "new" in that field – it will not include records with "new york", "new jersey", "something new", etc. — in other words, it doesn't run the search as %new%.