Quotes on USwitch different from actual supplier website

Saria

So I am currently looking to switch both my gas and electricity and am checking out various suppliers. However I have noticed that when I input the same usage details into USwitch, I seem to get different results (based on a monthly direct debit price). 

For example, it says that with Scottish Power I would pay £57.81 on their Super Saver April 2021 plan, but if I check on the Scottish Power website with the same details, the cheapest deal is £69.65. The same happened with E-on (£57.71 vs £68.83 on Fix Online Exclusive v28). And with EDF I'm shown a tariff which I can't even find on EDF's own website (the difference in price is significant though). 

So how is this possible? Is USwitch adding discounts or something to their prices? And if I go through them, how do I know it will actually be the cheapest rate? Or am I better off going through a cashback website myself? And why can I not see all the same tariffs on the supplier's website that USwitch seems to be showing me? 
Sorry for all the questions, I'm not used to switching, only done it once before! 

MWT

These days most suppliers have a range of tariffs, some of which are only open to specific channels like the various switching sites.

I would not pay much attention to the amounts they say you would pay or the savings you might make, just compare the actual unit costs and standing charges to see if they are better or worse than what you pay now. Also when using the switching sites look carefully for the option to display all tariffs, not just the ones they can switch you too as otherwise you are almost certainly not seeing the cheapest options.

Also consider that the absolute cheapest options from companies you have never heard of that have only been trading for a few months may not last and could end up dumping you onto expensive tariffs elsewhere when they go bust...

So choose wisely.