It's not necessarily "less accurate" data (it may or may not be), there is just less of it. A conventional ECG uses those 10 electrodes to view the heart from 1

It's not necessarily "less accurate" data (it may or may not be), there is just less of it. A conventional ECG uses those 10 electrodes to view the heart from 12 different "directions", which allows you to understand how impulses are moving throughout the heart as a whole. A watch only gets you one of those leads (Lead 1, specifically).