They do trigger at 100, normally. Was 79 the number on the circle around your profile picture or was it your number of agreements in your profile? And did the button that was available to you say Upgrade or Upgrade Next?
Other reasons why Upgrade numbers might be inconsistent are that Upgrades are also used as challenge rewards or sometimes they are reimbursed (as discussed here: Machine Learning Update and More Appeals! ). That final option would not apply to your first one, however.