all these cats had one experience in common

have 仅仅表示“ 有”
have sth in common  指“共有.”意即有着相同的.
如：
We have nothing in common.我们没有相同之处.
比较：
All these cats had one experience in common
所有这些猫都有着一个相同的经历
       All these cats had a common experience
        所有这些猫都有着一个平常的经历
 
 2.a,和one 貌似一样的意思,但one 强调数词概念,侧重“量”