This paper propose to analyse the effect of the shape factor that is used in plasma PIC deltaf codes to make interpolations between the grid and the particles positions. In deltaf codes, the total density fluctuates, even when it should be conserved. We show that, in some cases, the computed non-physical part of the particle kinetic energy fluctuations is dependent on those of the total density. We deduce a method that can reduce drastically the statistical fluctuations in the diagnostics of the kinetic energy.