Chatzos in the way that you calculate would be at solar noon (along the meridian) for the location that you are observing. Consider the meridian (the line along the longitude) at clock noon in the middle of the time zone. Thus, any location east of the center of the time zone would have the meridian before noon clock time. For example, Yerushalayim is approximately 20 minutes east of the center of its time zone. Thus, when chatzos is at noon in the center of the time zone, it would be at 11:40 AM clock time in Yerushalayim. Perhaps this is the situation that you are observing.
As an example, solar noon today in Jerusalem was 12:45 (Daylight Savings Time) which would be 11:45 AM standard time. Calculating chatzos shows a 14 hour day with sunrise at 5:46 AM which verifies the meridian.
What is Solar Noon
When Is Solar Noon?
In most places on Earth, solar noon does not happen at 12 o'clock.
The Earth's rotation slowly shifts the meridian
experiencing solar noon from east to west. In other words, solar noon
happens a little earlier in locations just east of you and a little
later in locations west of you.
Where's Sun right now?
Since our clocks are set according to time zones, civil time changes
abruptly as you move from one time zone to another, usually in 1-hour
increments. While this undeniably makes life easier for us, it does
not reflect the even movement of the Earth's rotation and the gradual
geographical progression of local solar time.
This means that clocks in the eastern part of each time zone show an
earlier time at solar noon than clocks near its western border. Even
if time zones were used the way they were once envisioned—where local
time is based on the solar time in the zone's center, with the time
zone extending 7.5 degrees of longitude to the west and to the east of
the center line—solar noon would occur at 11:30 (11:30 am) at the
eastern time zone border and at 12:30 (12:30 pm) at the western