South Africa 1s Stamp Missing Mountain Variety?interesting.

hi,this is a south africa 1s stamp.normally there are horses running with mountain behind them on the vignette but I discovered one similar stamp that the horses running without mountain behind them.i also can notice the black horse with less hair and white horse with no dot than the normal horses on the left stamp.i think the caused of this happend is probably due to the ink was too light during printing that time.am I right?im not sure whether it is a common variety or not till someone tell me about that.check it out and let me know what you think.thanks