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.
In marketing, lead generation is the initiation of consumer interest or enquiry into products or services of a business. Leads can be created for purposes such as list building, e-newsletter list acquisition or for sales leads. The methods for generating leads typically fall under the umbrella of advertising, but may also include non-paid sources such as organic search engine results or referrals from existing customers.