A neighbourhood split could expose that problem quickly. Keep the same 55-day calculation, then show it by area and by broad price band, including a band around €713,000. If one small segment accounts for most of the listings, readers will see why the combined number moved.
One final caveat: the lowest three-year total is not necessarily the best offer if its exit terms are materially worse. Since you already think a sale is possible during the fixed period, I’d compare both a “keep for three years” case and an “exit early” case. The second case may justify paying...