Guild Wars 2

I notice that the spool of weaving thread need 100 bolt of silk whereas, the mithrillium only ask 50 mithril ingot same for the thick elonian Cord and elder spirit residue.

So i was wondering if it was intended or if it just an error from the dev ?

Intended but I guess they never anticipated the impact it would have on the market when ascended armor was released.
They used it to drain some of the silk from the market, there were millions of scraps and bolts selling from 3c a silk and 10c a bolt prior to the ascended armor crafting release.