I had ECG last year and the findings was I have anteroseptal wall infarction. I went to my cardiologist and said it was all normal. What can this be?

Probably normal. The equipment itself could be the issue. The most common problem is the placement of the recording electrodes (stickers) on the skin. Just like any test in life, preparation is very important. If there is lingering concern, an echocardiogram may be helpful.
Limitation of ECG. Ecg is getting signal from your heart and is compared to a database. These are 70% accurate in predicting infarction. Echocardiogram are more sensitive and specific test. If you were evaluated by a cardiologist and had normal echocardiogram, i would not worry about the abnormal ecg. It is like having a positive urine pregnancy test (ECG) and a normal blood test and doctor evaluation (normal echo).
Multiple reasons. Many factors affect the ekg, the (relative) location of your heart in the chest, placement of the leads, and the equipment. What is felt to be an abnormal pattern for many people may be normal for you. As other colleagues has noted, additional information/tests, such an echocardiogram may sort this out.