As it is explained in the page, item enchantment levels are not a relation of a fixed amount of thaums per level, but the ratio of how many thaums are in the object divided by the maximum number of thaums for the object (based on weight).
For example, if we take the tiger fang, with a weight of 1 5/9, it can take a maximum of 8 thaums. If we add 1 thaum to the tiger fang we have an enchantment level of 1 / 8 * 100% = 12.5% or "slight octarine glow". If we have 2 thaums in the tiger fang 2 / 5 * 100% = 25% or "softly pulses in dull octarine shades". At 8 thaums it will be 100% and "radiates pure octarine brilliance". As you can see in this example, some levels are skipped.
For how to calculate the maximum number of thaums for a given weight see Enchantment#Enchantment_capacity.