Why the gym mirror lies to you about your progress
Judging progress by your daily reflection is a common but unreliable habit. How muscles look at any given moment depends on a bunch of factors that shift day to day and have nothing to do with real long-term progress: hydration level, how many carbs you ate the day before (which affects water retention and how full muscles look with glycogen), time of day, room lighting, and even the pose you're standing in.
These fluctuations create a situation where the same person, with the exact same actual body composition, can look noticeably different in the mirror on different days — not because anything physically changed, but because of short-term factors affecting how muscle size and definition appear visually. This is especially confusing early on, when real body changes are still minimal but water- and glycogen-driven fluctuations are already visible.
More reliable ways to track real progress: regular photos under consistent conditions (light, pose, time of day) every 2–4 weeks, tape-measure body circumference tracking, and of course objective gym numbers — working weights and reps, which aren't subject to random day-to-day noise.