Merriweather is used for the headers, Merriweather Sans is used for the body text. This hierarchy is based on a perfect fourth scale.
A perfect fourth scale sits between the medium and large scales. This scale serves to fit standard web sizes for both headings and paragraph text.
Archivo is used for both the headers and body text. This hierarchy is based on a perfect fifth scale.
A perfect fifth scale is a larger scale used for major eye catching designs. Since we're using a single typeface here, the major scale gives you the versatility for any scale of text.
Source Serif Pro is used for the headers, Montserrat is used for the body text. This hierarchy is based on a major third scale.
A major third scale is smaller compared to the perfect fourth and major fifth scales, but is still effective on standard sites.