The Leek XV, outclassed by Ireland last week (29-7), made the difference on a drop from Dan Biggar 10 minutes from the end, while his opponents were playing at 14 after a yellow card for Finn Russell (67th).
A few seconds before that, Dan Biggar had found the post on a well-placed penalty, but the forward, deemed voluntary, from the Scottish opener, negatively "crowned" an extremely disappointing performance from Russell.
At this stage of the match, it had become obvious that the difference would be in details between two teams who had had their moment of dominance in the first period.
Wales showed their ability to react after their difficult start last week in Ireland, taking the lead on two penalties from Dan Biggar (6-0, 7th).
But the Scots regained some offensive fluidity to score the first try of the match, at the end of the line, by Darcy Graham, who won his one-on-one against Louis Rees-Zammit to flatten in point (11th), and lead 14-9 at the half hour mark.
In front of their public, the Welsh however reacted before the break by scoring a try on a penetrating group by Tomas Francis to put the two teams equal at the break (14-14).
The second half was even tighter and the numerical inferiority in the last quarter of an hour weighed very heavily on Scotland who, once again, showed inconstancy, like their 2021 Tournament where, despite victories at Twickenham and the Stade de France, she only finished 4th.
Welsh winger Alex Cuthbert tries to escape Scotland's Pierr Schoeman in the Six Nations tournament in Cardiff, February 12, 2022 Geoff Caddick AFP
The penalty conceded in the 78th, when the first line replacing Welsh Dillon Lewis disputed a ball in a regrouping, shows the way that remains to be covered for the XV of the thistle to aim for the title.
The Welsh, still diminished by numerous absences of executives through injury, revived before going to challenge England at Twickenham in two weeks, while the Scots will receive France for a cleaver match.
© 2022 AFP