What is happening is the under the hood it is probably using Graphics.MeasureString(), from the documentation :
GDI+ adds a small amount (1/6 em) to each end of every string displayed. This 1/6 em allows >for glyphs with overhanging ends (such as italic 'f'), and also gives GDI+ a small amount >of leeway to help with grid fitting expansion.
The default action of DrawString will work against you in displaying adjacent runs:
Firstly the default StringFormat adds an extra 1/6 em at each end of each output;
Secondly, when grid fitted widths are less than designed, the string is allowed to contract
by up to an em.
To avoid these problems:
Always pass MeasureString and DrawString a StringFormat based on the typographic >StringFormat (GenericTypographic).
Set the Graphics TextRenderingHint to TextRenderingHintAntiAlias. This rendering method >uses anti-aliasing and sub-pixel glyph positioning to avoid the need for grid-fitting, and >is thus inherently resolution independent.
So it looks like you should be able to fix this using the correct StringFormat.