Council Tax Banding and Uplift Applied by Assessors (Scotland)

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I am not a Chartered Surveyor and I have very limited knowledge of Council Tax Valuation Legislation.  As far as I am aware the Council Tax Band is based on "the value of any domestic property shall be taken to be the amount which the property might reasonably have been expected to realise if it had been sold in the open market by a willing seller on 1st April 1991"   Assessors should look at sales evidence for a property roundabout this date.  If there is sales evidence then this should indicate the band - this is the primary evidence?  If there is no sales evidence then use evidence from comparable properties - I have noted from Tribunal Decisions in the Local Taxation Chamber (Scotland) that properties built 1990/91 have been used as "primary evidence". Also noted from Tribunal Decisions that the Assessor has quoted "there were no comparable properties sold on or around the tone date so direct comparison with open market sales evidence from 1991 was not possible. Therefore, the Respondent (Assessor) relied on uplift checks and rate per square metre calculations"  The number of rooms is not a material factor rather the total overall area is?  Why then would an Assessor ignore sales evidence from 1990 and apply an "uplift" and rate per square metre which pushes the property into a higher band?  Does anyone know anything about this?