The 18-week pledge only applies to England, but targets for hospital treatments are being routinely missed in other parts of the UK.
Rory Deighton, of the NHS Confederation, which represents NHS trusts, said: "The NHS is not one homogenous body but is made up of hundreds of separate organisations, each with their own distinct financial and operational challenges.
"This means that tackling care backlogs will be more difficult in some parts of the country than others – particularly if there are entrenched challenges, such as high levels of local deprivation."
Chris McCann, deputy chief executive at patient watchdog Healthwatch England, said the analysis showed there were "stark differences" for patients depending on where they live.
"Those responsible for monitoring NHS trusts must pay close attention to organisations that are not only performing poorly but actually deteriorating," he added.
A Department of Health and Social Care spokeswoman said the government had got the NHS on the "road to recovery" but there was still more to do.
She said investment was being made in services such as new surgical hubs and evening and weekend scanning, which would help.
And she said individual hospitals would be held to account for their performance.