Hi Sean,
Thanks for writing in! It’s simply because some browsers or different devices such as mobile devices does not support some font types, so it will render default font. Please refer to the following example.
@font-face {
font-family: 'MyWebFont';
src: url('webfont.eot'); /* IE9 Compat Modes */
src: url('webfont.eot?#iefix') format('embedded-opentype'), /* IE6-IE8 */
url('webfont.woff2') format('woff2'), /* Super Modern Browsers */
url('webfont.woff') format('woff'), /* Pretty Modern Browsers */
url('webfont.ttf') format('truetype'), /* Safari, Android, iOS */
url('webfont.svg#svgFontName') format('svg'); /* Legacy iOS */
}
So basically .ttf, .woff and .woff2 is needed so that there is a font file fallback when the browser does not support the font file.
However when I have checked your site on a Android device, your font rendered correctly and also I have tested it on chrome’s mobile testing tool. Perhaps you can mention your iOS version and also Safari browser versions as well to check your issue further.

Please make sure to clear all caches when testing your issue again (https://www.wpbeginner.com/beginners-guide/how-to-clear-your-cache-in-wordpress/).
Hope that helps.