No idea about salaries - and its probably one of those 'statistics' that doesnt actually hold water or is simply misleading - ie. LSE is more likely to attract overseas applicants or mature applicants or more wealthy students, simply because its 'in London'. And was this their first job, second job, 5 years into their career, and/or 'just the people we were able to contact', was it an average, a median, didi it include bonuses - and a whole bunch of unrealiable variables - who knows what this was actually comparing.
In anycase, 'potential salary' is actually a daft way to pick a degree or a career - drug dealing or extortion makes a tidy living but would you actually want to do that as a 'job'? Even if this was true about LSE, you have no inbuilt guarentee that you would earn 'more money'. And money doesnt guarentee you a happy or fulfilling life - think about it. Is that all you want to measure the value of your whole life agaiinst.