不一样.
since that 不是短语.其中的since 是自从,因为,既然的意思.例：It's been one year since that day.从那天开始算来已有一年了.
Since that result is based on a single criterion,由于那个结果基于一个单一的标准.
而 since then 是从那时以来,从那以后的意思.例：It is about a year since then.从那时到现在已有一年了.