Because the evidence is different in kind. Both use Form I-129 with the same base, anti-fraud and asylum-program fees on Form G-1055, but an L-1B requires the company's process to be documented and explained for a non-specialist reader, and a request for evidence, if issued, adds the cost of a response.
Same fee lines, heavier evidence and a higher request rate
USCIS charges the L classification one set of fees regardless of the subcategory: the Form I-129 base fee, the fraud-prevention and detection fee and the asylum-program fee at the full, reduced or waived rate depending on the employer's size, with premium processing available as an optional addition. Those figures come from the current Form G-1055. The difference is in what has to be assembled.
A managerial petition is built from organization charts, duty allocations and personnel documents that most companies already hold. A specialized-knowledge petition for a process requires the company to produce or refine the written procedure, document its development history, compare the beneficiary's knowledge with that of others, translate French-language manuals and present the evidence so an adjudicator without localization experience can follow it. A request for evidence is possible and, if issued, adds professional time.
Budget for that contingency, and consider premium processing so that any request arrives early enough to answer before the planned assignment date.