Dear M. Bakker,
no that would not have an impact on your ongoing survey, it's simply that the persons using https:// will be using SSL and the ones opening the http:// link won't be.
As for the reason why this isn't always being used, I'm not sure, this isn't exactly my are of expertise. First of all, SSL encryption is usually an optional feature for EFS-Survey, due to the costs associated with it. For installations hosting multiple clients, like all the Unipark installations, SSL is usually enabled, because there would be no alternative option to offer it for some clients, but not for others.
Also, according to Wikipedia, SSL connections require a higher amount of computing and are thus slower.