That is correct ... with any email marketing service. I use multiple email services just to see how the design lands in each one. ... also multiple browsers.
Website designers do the same thing with browsers, as the rendering is different for each one.
I've used FAA email campaign tool, TinyLetter, and Mad Mimi email services and see your findings from all of them.
You just learn over time how to position your content to balance the rendering in all of them. Go to my page and sign up to my email list to see how I do it ... I send out an email once per week.
Hope this is helpful to you Theresa. Enjoy your day.