Why do stats regarding the number of satellite launches differ?

[RonStat]

Why is it difficult to get an accurate figure regarding the number of satellites launched? I can understand why tracking agencies might disagree over the number of satellites in orbit but I don't really get why there's variation in the reporting of how many satellites have been launched. Are some launched unofficially or in secret? Very grateful to anyone who can help me understand!

 

[COLGeek]

You answered your own question. Not all things placed into orbit are disclosed.

 

[billslugg]

Jonathan's Space Report has the most accurate compendium of rocket launches. From what he has written I believe all countries abide by the UN, treaties, laws, etc whereby each launch must be announced. They don't have to say where the orbit is, or how many payloads there are.