GA SMA ON ILS APCH IN VFR CONDITIONS ALARMED AT CLOSENESS OF OBSTRUCTIONS ON FINAL APCH.

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CROSSED BASON OM WITH G/S NEEDLE CENTERED AND ALTIMETER READING CLOSE TO SPECIFIED 1894' MSL. ALTIMETER HAD A 50' ERROR AT TKOF FROM SAN (ALT READ 50' HIGH WITH CURRENT ALT SETTING ENTERED). AT ABOUT 1 NM FROM MM WITH G/S NEEDLE CENTERED AND ALTIMETER READING SLIGHTLY ABOVE 400' MSL; INSTR OBSERVED TREES; PWR POLES AND BLDGS ONLY ABOUT 100' BELOW ACFT (DH FOR ACH IS 295' MSL). AT SLIGHTLY BELOW 400' MSL INSTR DIRECTED STUDENT TO LEVEL OFF AND PROCEED VISUALLY FOR NORMAL LNDG. STUDENT WAS AS SURPRISED AS INSTR AT CLOSENESS OF OBSTACLES. FIRST OBSERVANCE OF 4 DEG VASI AFTER LEVEL OFF WAS RED OVER RED. TOA ILS G/S IS 3.4 DEGS. AFTER LNDG; PIC PROCEEDED DIRECTLY TO TWR TO DISCUSS WHAT CFI CONSIDERED A POTENTIALLY HAZARDOUS INCIDENT. TWR PERSONNEL STATED THAT ILS MONITOR LIGHTS INDICATED ALL WAS OK WITH THEIR EQUIP AND NOBODY ELSE HAD COMMENTED ON G/S PROBS. TWR PERSONNEL DID NOT REMEMBER OBSERVING OUR ACFT LOW AT 1.5 NM. PRIOR TO TKOF FROM TOA; ALTIMETER AGAIN NOTED READING 50' HIGH WITH CURRENT ALT SETTING. ACFT PROCEED DIRECTLY TO NEARBY LONG BEACH FOR PRACTICE ILS APCH WHERE ALL APPEARED NORMAL THROUGHOUT APCH. CAUSE OF APPARENT LOW ALT AT 1.5 NM FROM THRESHOLD NOT KNOWN. MISJUDGMENT OF VISUAL ALT BY INSTR AND STUDENT POSSIBLE. ANOMALY IN G/S ALSO A POSSIBILITY. RECOMMEND FURTHER INVESTIGATION. RECOMMEND ALSO PLTS ADD ANY ADVERSE ALTIMETER ERROR TO THE APCH DH OR COMPENSATE FOR THE ERROR WHEN ENTERING THE CURRENT ALTIMETER SETTING IN THE BAROMETER PRESSURE WINDOW.