Hmm that's interesting! Do you have any sources for that or are able to explain why? Thanks 
When I say science, I really mean physical sciences (theoretical chemistry, physics, mathematics etc). It is a trend that's becoming increasingly apparent in financial firms. Simply put, these firms know top positions are very quantitative in nature and they know such problem solving skills and quantitative background isn't enforced in a typical BComm degree. However, science kids have the appropriate training and the relevant skills to fill those positions, they are much more favoured.
In short, there's two "pathways" to top paying jobs in the financial industry:
1) Typical BComm: Need top top grades, good connections and networking, and a range of extracurricular activities to stand out. Why? Because BComm is the most common degree, much more grads apply to financial firms with a BComm degree than a science one. Even if those preqs are satisfied, you may still miss out on those top positions, simply because there are so many BComm grads.
2) Science/engineering degree (mathematics/physics/chemistry/biology): Again top grades are a must, but you have more room to distinguish yourself, you will be favoured for top quant jobs because a 'typical' BComm doesn't have the skills on their transcript to compete against you for those positions.