为什么都是以元音字母和辅音字母结尾的动词的进行式不一样

一个单词若是以重读绝对闭音节结尾（单个元音字母+单个辅音字母结尾）变成ing分词或ed分词时要双写最后的辅音字母.否则不需.
stop-stopping dig-digging swim-swimming
eat-eating boat-boating teach-teaching scratch-scratching
play- playing open-opening(注意其重读音节在前面)