Many radicals slightly change shapes depending on their position. It is no mistake, and it matters if you wish to be able to write the kanji correctly, but I don't think you need to concern yourself too much about remembering the exact shape: it will generally stick without much effort. A bit of caution, though: radicals, when changing shapes, sometimes change stroke count and stroke order! Heisig generally doesn't go into much detail about those different stroke orders; you may have to look at the animated stroke diagrams on wwwjdic to check those.