Trunks Special Saga - (Different Time Line)
The Android's power levels are based on the fact that Trunks states the Androids in the Future timeline are much weaker than the ones he encounters in the present timeline. Plus all the Z Warriors are a lot stronger in the present time Line.
At this point all the Z warriors are dead in the Future timeline. Goku died due to heart attack. All the other Z warrior including Vegeta were killed in combat with the Androids. The only two Z warriors survived in the Future timeline Gohan and Trunks.. (Bulma is alive). So Super-Saiyan Gohan trained Trunks. But Trunks was not ready to fight the Androids so Super-Saiyan Gohan goes to fight the Androids but is killed. Trunks lears that Super-Saiyan Gohan is dead, he's then filled with rage turns Super-Saiyan Trunks. But he is still no match for both Androids #17 and #18. The Androids have made earth a living hell.

Ruthless Androids Destroy The Earth.
Super-Saiyan Gohan: 22,000,000
Trunks: 4,000,000
SSj Gohan and Trunks train very hard.
Super-Saiyan Gohan: 25,000,000
Trunks: 5,000,000
Android 17: 23,000,000
Android 18: 22,500,000
SSj Gohan is killed in combat. Trunks truns Super-Saiyan.
Super-Saiyan Trunks: 18,000,000
Android 17: 23,000,000
Android 18: 22,500,000
Even in his Super-Saiyan form he is no match for the Androids. So his Mom Bulma makes a time machine. Trunks uses it to go back in time where Freeza Saga ended and change the future...

Freeza Saga - (takes place at end of the Saga)

Arrival of Future Trunks
Trunks comes back in time using the time machine his mom made. After killing Freeza Bot and King Cold. Trunks talks to Goku for a long time and then challenged him to a friendly combat. He then tell him he's from the future and gives Goku the medicine for his heart disease. Vegeta not knowing Trunks is his future son is realy mad because he finds out Trunks is a Super Saiyan. Keep in mind that Goku was trainning in a different planet and has now mastered his Super Saiyan powers and is lot stronger than before. Also, Powerlevels are hard to calculate in Super-Saiyan level.

Trunks: 15,000,000 (basic level)
Frieza Bot: 11,000,000
King Cold: 13,000,000
SSj Trunks: 18,000,000
Son Goku : 16,000,000 (basic level)
SSj Goku: 22,000,000 aprox.