Lean mass is made up of more than just muscle.
It includes bones, organs, muscle, blood supply, muscle, tendons, and water.
Many of these (bones and organs) are pretty much fixed in weight.
Muscle and tendons can increase over time, as can the volume of blood for people who exercise regularly and intensely. Increases do happen, but generally only slowly.
Water is the biggest variable, and can change from day to day (or even hour to hour) depending on your hydration levels. A single 8 oz glass of water weighs half a pound.
It is also very common for your muscles to retain water when you start/increase an exercise program. This is water, rather than muscle tissue, but it is part of your lean mass.
A 3-5 pound increase in lean mass is not uncommon at the start of an exercise program, but this is largely a one-off effect. The fact that this matches your fat loss over a month is just a coincidence of timing - over time you can expect your fat loss to continue, while further increases in your lean mass will be very gradual (if any).
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
| current weight: 178.0