Your body converts food to energy-calories. It burns this calorie to lift the weight, and recover from your workouts. Most guys need at least 3000kcal/day to gain strength and build muscle on the strong lift 5×5. Skinny guys with the first metabolism may need to eat even more. Here’s why building muscle is low on your body’s priority list. If there’s a shortage of food, your body will use it for critical tasks first. So you can not recover well from a caloric deficit. And if you can not recover well, you can not add weight next workout. You miss reps which means you can not progress.
Eating maintenance calories is better but not ideal. You are not trying to maintain your situation on stronglift5x5 after all. You are trying to improve it by gaining strength and muscle. Eating over maintenance ensures there’s no food shortage that hinders recovery between workouts. Good calorie calculators will suggest guys 16kcl/lb for maintenance. If you weight 75kg/165lb that’s 2640kcal. But again, you are not trying to maintain but improve. And adding weight every workout is hard work. it, therefore, makes sense to eat more.
Unfortunately, it is hard to build muscle without gaining any fat. You heave to eat more to build muscle. But you need to eat less to lose fat. You have to eat more to build muscle. But you need to eat less to lose fat. These goals contradict. If you try doing both, you will either end up eating too little to build muscle or too much to lose fat. This is why bodybuilders traditionally alternate muscle gaining and fat loss phases. They eat more food during the bulk, but less food during the cut. This is the simplest way to build muscle without gaining fat that actually works if you are not obese, have not trained before, and do not use drugs.
Obese guys can indeed build muscle while losing fat when they start lifting. Their bodies use their fat reserves to build muscle. They build strength and muscle faster without needing as much food. And since muscle is denser than fat, they end up looking slimmer at the same body weight. People who have lifted before can also build muscle while losing fat. Thanks to muscle memory you can regain lost muscle and strength faster after a long break. If I quit lifting and resume a year letter, I have rebuild strength and muscle faster then it took the first time- while leaning out.
If you use drugs or have great genetic, then you can do things that naturals and mere mortals can not. But most people who try to build muscle while losing fat end up spinning their wheels. They do not progress because too much food to lose fat not enough to build muscle. You can get away with eating a calorie deficit the first weeks of Strong lift 5×5. If you start light, the weights will increase and you will gain some muscle. But the heavier the weights get, the bigger the stress, and the bigger the recovery need. you will need to eat more to keep progressing.