The cycle is the main reason the two numbers separate. Assessed values are set from a base date and held across a revaluation period, so the whole market can move while your notice sits still. When the next revaluation lands, assessments jump in one step to catch up, which is why the notice can feel wildly wrong in the last year of a cycle and roughly reasonable in the first.
Mass appraisal is the second reason. The City values thousands of properties from recorded attributes rather than by visiting each one, which is the only practical way to assess a whole city. It is a fair method for distributing a tax burden and a poor method for pricing one specific house, because the model cannot know your furnace is two years old and your neighbour's is twenty five.
If you believe the assessment itself is wrong, the remedy is an appeal to the Board of Revision within the period printed on the notice, not a phone call to an agent. For what the house would actually sell for, you need current comparables and someone who has seen the improvements, which is what Joel Dyck's hand-reviewed valuation provides.