Noticed while testing #26467 (comment).
WALLET_INCREMENTAL_RELAY_FEE was introduced in #9615 presumably to account for a situation where the users mempool was less full than that of most peers. Bumping by exactly DEFAULT_INCREMENTAL_RELAY_FEE might then cause the bumped transaction to not get relayed.
In addition, because the GUI doesn’t let the user specify the new fee rate yet (https://github.com/bitcoin-core/gui/issues/647), it would be very annoying to have to bump 20 times to increment by 20 sat/vbyte.
However, when you use the bumpfee RPC and manually specify fee_rate this is a needless restriction.