Right. I answered too quickly. The starting page number is set in the “special” break. I edited my comment.
A page has an “intrinsic” page number assigned when it is created (i.e. when text requires a new page after the current page is filled). This page number is the next sequential number. You can’t modify this incrementation rule. Your only way is to break incrementation with a “special” break where you force the page number after the break.
You can do this without changing the page style (in fact, you specify the same page style).
Offsets enter in play when you want to reference a page, but this is tricky and quite “dangerous”. Your reference is usually linked to an “object” like a heading, bookmark, reference or image, etc. The reference is located in a page. You can then request another page relative to this reference: this is the offset (0 for no change, +1 for next page, -1 for previous page, …). You land then on another page from which you extract the “intrinsic” number.
When the offset page exists you see no difference with the forced starting number feature. But, when you’re near the “ends” of a page sequence, the final offset page may not exist, e.g. you’re on the last page and you asked for offset +1. There is no such page and the number returned is blank!
Regarding TOC, the engine captures the heading page number with an offset of 0 (page on which the heading occurs). The TOC displays the “intrinsic” page number.
When you insert a page number, you do this with a field. The page number field is very general, used in many contexts. Writer does not know what you have in mind (do you want to simply number a page with its “intrinsic” number or are your referencing another page to “quote” its number?) and has no reason to restrain the expressive power of the field.
Offset is not a compensation for pages to be ignored. Writer is more feature-complete than Word. Its model is based on a well-designed abstraction (perfectible probably, but much better than the one in Word).