NAP stands for name, address, phone number. Consistency means those three things are written the exact same way everywhere they appear: your website, your Google Business Profile, every directory, your invoices and your van.
What counts as a difference
More than most people expect. Kingfisher Roofing and Kingfisher Roofing Ltd are two different names. 01614 567890 and 0161 456 7890 are the same number to you and two strings of characters to a computer. Unit 4 and Unit 4a are different addresses. So are Road and Rd.
None of this means Google is stupid. It is reasonably good at matching near misses. It just gets less certain, and certainty is the thing you are buying.
How the mess usually starts
Almost nobody sets out to have four versions of their details online. It happens because a listing gets made in a hurry, a partner writes your name their own way, or you move premises and update the five places you remember.
The other common cause is tracking numbers. A directory gives you a number that forwards to your real one so it can count your calls. It is now showing an address and a phone number that exist nowhere else, and your details have quietly split in two.
A worked example
A builder in Wakefield trades as Hemsley Building Services. On Google it is Hemsley Building Services. On one directory it is Hemsley Builders. On a second it is Hemsley Building Services Ltd Wakefield, because whoever typed it added the town to help.
A homeowner who has been given the name by a neighbour searches it, sees three near identical entries, and cannot tell if they are the same firm or three rivals. The neighbour is not there to ask. That is a lost call that never shows up in any report.