some wifi have quality as 'Quality:38  Signal level:0  Noise level:0'

I'm not sure why, but I have this:
root@tortue:~/wifi# iwlist  scanning | grep Quality
                    Quality=100/100  Signal level=100/100  
                    Quality=100/100  Signal level=100/100  
                    Quality=15/100  Signal level=26/100  
                    Quality=0/100  Signal level=42/100  
                    Quality=100/100  Signal level=44/100  
                    Quality=25/100  Signal level=23/100  
                    Quality=88/100  Signal level=42/100  
                    Quality=100/100  Signal level=72/100  
                    Quality=100/100  Signal level=74/100  
                    Quality=100/100  Signal level=76/100  
                    Quality=100/100  Signal level=67/100  
                    Quality=40/100  Signal level=10/100  
                    Quality=100/100  Signal level=100/100  
                    Quality=100/100  Signal level=50/100  
                    Quality:38  Signal level:0  Noise level:0
                    Quality:36  Signal level:0  Noise level:0
                    Quality=100/100  Signal level=10/100  
                    Quality:59  Signal level:0  Noise level:0
                    Quality=101/100  Signal level=43/100