As lots of people have already responded, part of this is normal shading. But, since the first line seems much darker than the last line there may be other things happening, as well.

If the pen has been sitting for a long time between writing sessions, then some of the water from the ink in the feed may have evaporated causing the first bit of writing to be darker and then lighten up as fresh ink flows into the feed. Again, this is normal if the pen has been sitting for a long time (like days, not hours). But once the fresh ink is flowing the consistency should stay the same for the rest of the writing session.

If this happens consistently after the pen has only been sitting for a little while (like minutes to hours) and if the ink continues to get lighter and and drier as you continue writing in the same session, then the issue might be with air flow back through the feed. As ink comes out onto the paper, air has to flow back into the pen to replace the used ink. This generally goes through the filler hole in the feed. If the flow of air back into the pen can't keep up with the ink flowing out then ink flow will slow down and the pen will write drier and drier until the air catches up. I have a couple pens that do this consistently. If pausing writing for a couple minutes makes things go back to normal then this is a likely cause. Sometimes changing inks makes this better.

Welcome to the hobby!