What JimLad said ^^^
But also, a major problem is that for 20+ years the Services have not been required to Quantify their requirement. For example, the RN stopped doing it as a matter of policy (for aircraft and aircraft equipment) in 1988 (LTC 89), but a few old hands continued meeting the regs* until they left in 1995; which explains variations in efficiency between projects. ( I never knew the RAF to do it at all!!).
* permanent LTC Instructions, never rescinded.
If you don't quantify, you can't cost accurately. Hence, "cost overruns" which are actually nothing of the sort, because the requirement has never been properly costed in the first place, so you have no valid baseline.
In 1995 the RN formally advised MoD(PE) it was not for them to state their requirement, it was for the MoD(PE) project manager. However, this was becoming increasingly impossible, because fewer and fewer PMs had worked in this lower level HQ staff post before being promoted; mainly because they were by now primarily direct entrants and had skipped these lower grades (e.g. Requirement Managers, ILSMs) and had no relevant experience.
I think most here who have done a Staff job in "acquisition" will understand the above and relate to it.