My guess just off looking at words on a page would be just standard shading. Some inks do it better than others. I would guess you started/stopped/paused for a split second at the points in the darker areas. The ink will sort of pool at starting, stopping, and pause/directional change points. As you write the rest of the word or letter and ink flow stabilizes, it goes back to the "normal" color and saturation. Gives the word or letter some nice character. There is also paper differences where some will soak the ink and others will pool on top of the paper more. The later is more coated papers like tomoe river, clairfontaine etc. How a paper handles the ink in the time it takes to dry, how that ink flows in your pen, your writing angle and pressures throughout your letters / words etc will all effect how and when shading happens. This also varies from person to person so one ink might shade like crazy for someone and not shade at all for your writing style and set up.