Each factor is measured at two levels, which can be coded as −1 when the factor is at its low level (L) and +1 when the factor is at its high level (H). The coded variable can be defined as follows (suppose that there is a mathematical equation here) where x and r are coded as variable and real variable, respectively. h and l represent high level and low level of factor. Factors and their levels are shown in tables 1 ... . Why can't we use "encode" instead of "code"?
I suspect that this just happens to be the way the jargon has evolved, MNB. A few people started using the verb 'code' in this precisely defined way, and others in the field followed suit.
I recommend you do the same.
Whilst it's true that 'code' and 'encode' have an appreciable overlap of meaning in natural language, you must realise that the example you have given is not taken from natural language.
The source is a paper: "Mehrdad Mohammadi, Fariborz Jolai, Reza Tavakkoli-Moghaddam, Solving a new stochastic multi-mode p-hub covering location problem considering risk by a novel multi-objective algorithm, International Journal of mathematical modelling, DOI: http://dx.doi.org/10.1016/j.apm.2013.05.063."