Laser is very good on hair but top lip hair is very stubborn.
Everyone has a normal for them amount of hair which is partly to do with your natural hormonal balance. Lip hair is affected by your sex hormones, and these fluctuate throughout your lifetime so this area is trickier to laser effectively than say arm hair.
Also everyone has potential hair follicle cells deep under the skin and these can get activated at any time, including stimulation by imbalances/changes in your sex hormones.
So laser kills hairs very effectively but it has no effect on future hairs which haven't developed from follicles and it doesn't affect your sex hormones which create the conditions which cause hairs to grow/develop.
If your top lip hair is very fine or not strongly contrasted against your skin then the hairs might not "soak up" a lethal dose of light, so sometimes the hair can be strangely unaffected. The skill/experience of the therapist and the effacy of the machine (good quality, not in need of a service, capable of adjustment to the closest match to your skin/hair type) also makes a difference. You do get what you pay for.
I'm naturally quite hairy and had fairly fine, but dark top lip hair. I had laser 20 years ago and now that I'm post menopause and destined to turn into a bearded lady, I'm still less hairy than in my 20's, but my top lip isn't bare. I've still got hair which is fine and slow growing and I remove it every now and then. It doesn't bother me now and hasn't ever since I had laser. I get a top up of a few sessions every 5 years or so and I'm overdue, but still not looking like a Mexican bandit.