Why it differs from solar time
Earth orbits the Sun while rotating. After one turn relative to distant directions, it must rotate a little farther for the Sun to return to the same meridian, making the mean solar day longer.
JPL lists a mean sidereal day of about 86,164.09 seconds, roughly 3 minutes 56 seconds below 86,400 seconds. Stars therefore return to similar local positions about four minutes earlier on successive dates.
The local meridian supplies the observing reference
Local sidereal time relates the equinox direction to the observer’s meridian, making site longitude part of the definition. One civil-clock reading does not specify the same local sidereal time at every longitude.
Combining a target’s RA with consistently defined local sidereal time yields its hour angle. At meridian transit the hour angle is zero, directly connecting the catalogue direction with local geometry.
Mean and apparent conventions
Precession, nutation and chosen reference directions matter in precise definitions. Mean and apparent sidereal time require conventions consistent with the RA and equator being used.
Horizons explicitly labels local apparent sidereal time and hour angle for topocentric observations. Practical pointing needs a site-, time- and frame-specific calculation rather than extrapolating a four-minute-per-day rule.