1. The scouters don't go high enough and levels can be hidden from them and the levels are so high no one can read them using a device people just use their senses
2. Akira Toriyama decided that in order to be more flexible and to vary the outcomes of the battles not to write down power levels
Trunks (supressed): 5
Super Saiyan Trunks: 17,000,000
King Cold's henchmen: anywhere between 3,000 and 7,000 each
Cyborg Frieza: 12,500,000
King Cold: 13,000,000
Well what can I say Trunks shows up goes super saiyan and totally annihilates Frieza, King Cold and their henchmen. Frieza probably gained a slight power boost from the mechanical parts on his body while the Z warriors state that King Cold is stronger than Frieza (despite that he looks like he is in form
2) and King Cold's henchmen are probably at 3,000
The Arrival of Goku
Super Saiyan Goku: 20,000,000
Super Saiyan Trunks: 17,000,000
Vegeta: 960,000
Piccolo: 1,000,000
Tien: 91,000
Gohan: 70,000
Yamcha: 63,000
Krillin: 54,000
Chaozu; 28,000
Finally after wondering where in the universe he was Goku returns and faces off against Trunks and wins. Goku is therefore stronger than Trunks while the others I based on the Garlic Junior/Frieza saga levels and the level of training. Characters like Piccolo, Vegeta, Tien and Chaozu are always seen training meaning their levels will increase faster than characters who only train every now and then like Yamcha, Gohan and Krillin (whi is een practicing his Kamehameha at the beginning of Frieza's Counterattack)
Appearance of the Androids
Super Saiyan Goku: 25,000,000
Piccolo: 8,000,000
Gohan: 5,000,000
Tien: 4,500,000
Krillin: 4,000,000
Yamcha: 3,000,000
Yajirobe: 1,150
After three years of training awaiting the androids they finally arrive. The levels are based on the training the character did when compared to their levels before and the known level of training they did eg Gohan trained hard while Yamcha also trained but kept slacking off
Yamcha vs Android 20
Yamcha: 3,000,000
Android 20: 5,000,0000
Android 20 (after absorbing Yamcha): 5,100,000
Yamcha is the first one to find the androids even though he doesn't realise it at first. Android 20 probably gained a slight power boost after absorbing Yamcha.
Goku and Vegeta vs. Android 19
Super Saiyan Goku (sick): <10,000,000 (drops as he gets sicker)
Vegeta: 27,000,000
Android 19 (before fight): 10,000,000
Android 19 (after absorbing Goku and Goku's blasts): 10,000,0000
Android 19 (after absorbing Vegeta): 10,200,000
Goku takes on Android 19 but he is unable to destroy him because of his heart disease but Vegeta steps in and takes him down! If your wondering why Android 19's power level is the same after absorbing Goku and blasts its because when he did this he got back the energy he had lost while fighting Goku. He proabably gained a slight power increase after absorbing Vegeta
Piccolo vs. Android 20
Piccolo: 8,000,000
Android 20: 3,000,000
Android 20 (after absorbing Vegeta's blast): 5,000,000
Android 20 (after absorbing Piccolo): 7,000,000
After absorbing Vegeta's blast and Piccolo, Android 20 finds he is still no match for Piccolo. You may notice that 20's power level goes down after absorbing Yamcha this is because he used up the energy after flying etc
Android 17 vs. Android 20
Android 17: 35,000,000
Android 20: 5,000,000
In a final resistance act Android 17 steals and crushes the remote controller and then kicks off Dr Gero's (#20) head and then crushes it ending the doctors life
Vegeta vs. Android 18
Super Saiyan Vegeta: 27,000,000
Android 18: 35,000,000
Vegeta is dealt a big blow to his ego as Android 18 brakes his arm and K"oes him. Poor Vegeta... losing to a girl shame on him...
Piccolo, Trunks and Tien vs. Android 18
Super Saiyan Trunks: 20,000,000
Piccolo: 8,000,000
Tien: 4,500,000
Android 18: 35,000,000
The Z fighters have no chance against the more powerful Android 18 and her brother #17 are all easily k'oed but are revived with senzu beans thanks to Krillin