Why did my Neto system skip an online order number? EG went from N127 to N129.
This is totally normal. Internally, Neto allocates the order number fairly early in the checkout process. If the customer doesn't finish checkout or their payment declines then the order isn't actually created and that number is skipped.
You might find the Checkout Error report interesting (Reports > Abandoned Checkout Log). This shows some of the reasons checkouts have failed and you might see your missing order numbers in there. These entries are usually nothing to be concerned about, they'll be things like credit cards declining or customers being denied finance through Afterpay/Zip.