Variation in fonts with composing email

I am using the free version of eMClient, version 7.1.33101.0, on Windows 8.1
I have set my default settings to use Calibri, 12 , black font.

However, I noticed a problem.

When composing multiple line email, the font type, size varies. Atleast two varieties.  I have a personalized HTML email signature also set up to compose. So does that make a different.