underreckoning

/ˌʌndərˈrekənɪŋ/
noun
  1. An estimate or calculation that is too low; the act of underestimating something.
    • The project failed due to an underreckoning of the time needed.
    • An underreckoning of the population led to a shortage of school supplies.
    • The budget suffered from a serious underreckoning of the costs.