Last year the offer radius was 3.1 miles and has been similar to that for the last couple of years. We are 1.6 miles away - walking distance
Then why are you worrying? Provided the 3.1mile figure you gave was for the distance criteria only (not siblings, etc), then you are well within it. Unless there is an unusual event, such as a massive sibling bulge, a reduction in the admission numbers, a huge house-building programme coming online, and/or several bulge classes graduating from local primary schools, you'll probably be fine. Apart from the sibling cohort size, all of those things can be checked in advance of applying, so you can reassure yourself.
why will the distance criteria change?
The answer to your question as worded is that if the criteria have changed for 2018 relative to 2017 then that will be reflected in the wording of the policy. The policy for 2018 entry will now be fixed so you can check. (But if you're applying for later years, then watch out for the school consulting on changes in January/February of the year before you apply - the timetable for change is statutory and set out in the Admissions Code).
If you meant "why does the distance fluctuate from year to year even if the criteria don't change?" then that is because of the natural variation in the numbers of siblings, bulge classes, population size etc. It can also be because of a change in Ofsted ranking, positive or negative.
However, if you meant "why does the distance increase between March and September" then that is because of waiting list movement.
If you live in an area where everyone gets 6 preferences rather than 3, and/or an area where a lot of families give up their state offer (even if it is their first preference) to go private, then it is likely to increase fairly significantly.
Our LA sends out offers, and then re-offers of higher preferences, in "waves", the first being on National Offer Day, the next being shortly after the acceptance deadline 2 weeks later, followed by a trickle of further offers as various private school deposit deadlines are passed, and another moderately sized wave after the LA-wide transition day in July (after which no-shows are contacted to see if they still want their places). There are then more trickles over the summer holidays, and a final "wash-up" at the beginning of September when most schools have one or two "no-shows", whose places are offered out to their waiting list.
The most popular schools are desperate to fill all their places by the "census" day in late September, which determines their funding for the year. The least popular schools have a gradually diminishing cohort, and usually lots of spaces by September.
Our local uniform shops routinely tell people to leave tags on their newly bought uniforms in case they get a higher preference offer, as they're used to making the swap.
Yet, none of this is visible in the admissions brochure which just provides "last distance offered on National Offer Day" for the previous year and makes good and outstanding schools look a lot less attainable than they actually are.