feedforwordnet generate various performace for the same neuros

i have 22 features for 50 images (25 benigne and 25 malignant) i tried to classify them by nn and (feedforowardnet) command ,my problem is when i determine the number of neurons (15) get accuracy 90% and mse .09, when i repeate the command ,by the same neuons (15) the results was chaged completely (accuracy 92) and mse .1 ,i wana know what the wrong is thanks