The likely answer is to do with audience biases. A survey conducted by an email marketing provider is almost certainly going to have different results to one conducted by PPC management tool, as their audiences have different skillsets and biases, skewing the results of their sample. As such, we should take the specific ranking of different strategies in these studies with a pinch of salt.
Online surveys: Consumers are asked to complete a survey, including their demographic information and product and lifestyle interests. This information is used as a sales lead for advertisers, who purchase the consumer's information if provided. The consumer may 'opt-in' to receive correspondence from the advertiser and is therefore considered a qualified lead.