Designing a computer application to whatever field implies solving a number of problems, [color=red]mostly deriving from the variability which typically characterizes instances of the same real-world problem
It is saying that there is a lot of variability inherent in real-world situations (e.g. conditions, responses etc. can change unpredictably). When designing a computer application to simulate, predict, analyse etc. a real-world situation, this variability causes difficulties.
The sentence is not very well written: designing a computer application “to a field” doesn’t make proper sense.