Reconstructing an event is not as easy as reading off values from a voltmeter.
The detector itself gives a collection of digital and analog signals, corresponding to hits and (for some subdetectors) energy deposition.
Then you have to look for tracks / showers in those signals. This process is not perfect - sometimes uncorrelated hits look like a track without an actual particle there, sometimes you miss so many hits of a particle that you do not reconstruct it as track, sometimes a real particle gets assigned wrong hits as part of the track, sometimes the hits are so dense that tracking does not give proper results at all. All those options will give wrong results.
You get a similar issue in calorimeters: the overlap of multiple showers is a very common effect.
Once you assigned those hits to tracks, you can fit for particle properties like the energy, the transverse momentum, the origin of the particle in the interaction region and so on. From those tracks, you can find the primary vertices in the event - points where multiple tracks go through.
Depending on the analysis, you might want to re-adjust the track fits now to let multiple tracks come from a common primary vertex or to let individual tracks come from a specific vertex. This is especially important for photons and neutral kaons, as you just see them in the calorimeters, without tracking in the inner part of the detector.
Tracking also has to take alignment into account, for example: the detector elements are never exactly at the design positions, because construction of a meter-sized tracker is not precise down to the micrometer-level. The deviations have been measured and they constantly get updated, together with other improvements - reconstructing tracks again later ("reprocessing", often done a year after the data has been taken) will change all those values a bit and make them more precise.
Calorimeters are very complex devices and need a lot of calibration to get realistic results. The ratio of energy deposition in the absorber to energy deposition in the active material both for hadronic and electromagnetic showers, the fraction of energy escaping as neutrinos or muons, the responses of individual active elements to scintillation light and corrections for dead elements (or infrastructure), just to name a few things.
This is just a very short part of a long list of effects that are considered in the reconstruction. Sometimes there is no "best" way to handle things and different analyses use different methods.I am sure you did not have to consider all those things. But someone else did. And your numbers and their correct interpretation depend on the way this was done.
a whole spectrum of nonsense results.
