I have asked that question and was told that the denominations are essentially the same, except that the above-mentioned split occurred over the way in which a FCoS minister was being disciplined (or not being disciplined as the case may be). That may not have been the only issue, but was perhaps the proverbial straw which broke the camel's back.
The FCC were unhappy with how allegations against a Free Church College professor were handled.
The FCC isn't keen on the new version of the Psalms ("Sing Psalms"). I don't know if only the KJV is approved of. I don't know if Christmas is kept by FCC families. FCC ministers eschew theonomy but it is allowed for office bearers if they so wish.
The FPC tends to follow closed communion, apart from one or two denominational exceptions, only allows the KJV in the pulpit, tends to preach against Christmas, and women must wear hats.
In the FC there is open communion, the KJV, ESV, NKJV and NIV are used in different pulpits, there tends to be little preaching against Christmas, and women choose to wear hats or not.
I'm sure someone will correct me if I've got any of this wrong.
Differences have to be emphasised to help justify denominations.
Maybe some other - more important - differences can be thought of.
I could be wrong, but I would think that the recent controversial issue regarding the use of hymns in the FCoS would be a different (as in I do not believe the FCoS(C) would allow them). Please feel free to correct me if I am wrong on this.
Most Free Church people wouldn't allow them in a service, either. But they have been allowed at home in the FC, FPC, FCC and APC. I've heard that some Reformed Presbyterian CofI folks believe it is always a sin to sing a hymn, but it is not an expression that I've heard in the Scottish denominations.
Maybe this permission to sing hymns can be traced to the presence of other songs in Scripture apart from the Psalms.
You've got to remember that the USA is 60x the population of Scotland (300 million vs 5 million) which helps to make our denominations look bigger.
If we weren't in a micro-denomination we'd be in an independent church or one of the mixed Presbyterian denominations, the United Free CofS, or the CofS. Also unthinkable would be the Scottish Episcopal Church or the RCC.