This is not true. It's quite close to the center but the reason it's off to one side is due to how the heart develops. The heart starts out as a set of two straight tubes which fuse together. The heart is actually beating in this state as a straight tube. The heart's final shape is due to a twisting of this tube. You can visualize this with a string. Grab it with each hand about an inch apart. Then gently twist it as you bring your hands a little closer together. You'll see that the string will bulge out to the side. Normally the heart will bulge out to the left which is why the left lung has one less lobe than the right.

Normally, both the left and right ventricles point to the left, about +60° down from horizontal. But it's relatively common to see anything from -30 to +100° from a horizontal line to the left. This is called the heart's axis and it can be determined by ECG. It is true that in something like high blood pressure for example, the left ventricle enlarges so much that this axis will change to <-30° and similarly with high pulmonary pressure causing right ventricle enlargement to shift the axis >+100°. There are also situations, e.g. dextrocardia where the heart is switched to the right and situs inversus where all the internal organs are on the opposite side. But normally the heart is on the left and points down and to the left.