Why developers score lower on code
A typical prose passage is almost entirely lowercase letters with the occasional comma. A line of code might contain parentheses, braces, angle brackets, colons, arrows and underscores in a single statement. Every one of those needs either a reach to the number row or a shift chord, and most people never practise them deliberately. A drop of 30 to 40% from your prose speed is completely normal here.
The symbols worth drilling
The usual culprits are the bracket pairs, the underscore, the pipe, the ampersand and the arrow sequences like => and ->. Each one is a shift chord or a long reach. Drill them in context rather than in isolation: your hands need to learn the transition into and out of the symbol, not just the key itself.
Editors hide the problem
Autocomplete, snippets and bracket pairing mean you rarely type the hardest sequences in full during real work, so the weakness never surfaces and never improves. A code typing test is one of the few places you actually type every character, which is exactly what makes it useful practice.