with regards to your question:
(most) database tables – wppizza related or not – will have auto (increment) id’s so when a new row gets inserted the id increases.
in relation to wppizza, not all rows added to the order table will subsequently end up being a completed order, so it is more likely than not in fact that db id’s (invoice ids) will *not* be consecutive – though they will be sequential and unique.

regarding the the rest

anyone who knows even only the most basic things about databases can change id numbers in whichever way he/she likes.
saying “missing order-id’s could be classified as fraud ” is just nonsense in my opinion and a “red herring”.

If I want to commit fraud, I commit fraud, if I don’t, I don’t. Furthermore, it’s for the accuser to prove that the business committed fraud, not for the business to prove that it did not.
(or – to use your words – if I want to “fake” things, it will never be a problem; this is not a wppizza specific issue in any event )

lastly, it is also often stated that invoice number must be consecutive (i.e 1,2,3,4,5) . This is simply untrue(*)

(*)Disclaimer: the above is related to most countries I know of and is my personal opinion based on the knowledge I have as of writing this. I am not a lawyer or tax adviser and I cannot be 100% sure of course for all countries as I do not know the tax laws of each and every country on the planet. If in doubt, please consult a professional on these matters