Understanding Society User Support

Hi - In the User Guide it says "At the household level, w_fihhmnnet1_dv is the net household monthly income. It is the sum of net monthly incomes from all household members (including proxies and within household non-respondents, see w_fimnnet_dv). It can be decomposed into the six subcomponents: net labour income (w_fihhmnlabnet_dv), miscellaneous income (w_fihhmnmisc_dv), private benefit income (w_fihhmnprben_dv), investment income (w_fihhmninv_dv), pension income (w_fihhmnpen_dv), and social benefit income (w_fihhmnsben_dv)." Should then the 6 income components (w_fihhmnlabnet_dv, w_fihhmnmisc_dv, w_fihhmnprben_dv, w_fihhmninv_dv, w_fihhmnpen_dv, w_fihhmnsben_dv) sum to equal w_fihhmnnet1_dv? When using the hhresp file, this only seems to happen for about 80% of households (give or take a few £s difference). The difference for some households is large. I ask as I'm trying to analyse changes in total household income over time and to look at which components of income are causing this change. Thanks for your help. - Matt