The American Community Survey (ACS) 5‑year estimates reflect the average characteristics for the entire five‑year period. They do not represent a single year in the middle of that period.
If the measure you’re looking at has been stable or has changed at a constant rate over those five years, then the 5‑year estimate can be a reasonable stand‑in for the middle year. But if things have been changing unevenly, the 5‑year estimate won’t accurately reflect the midpoint.
Read more about this in Period Estimates in the American Community Survey.