Its muddy the water time
Ignitions do NOT have a smooth current draw. They take a really big bite of power just after the plug fires and the cap is empty. As the HV cap charges the current decreases. Now all this happens in milliseconds and most MA meters won't show it, but an O'scope will. This makes a big noise spike on the battery leads that is equal to engine rpm. This is one of the major "RFI" sources and why we say to separate your batteries.
Also this power surge can swamp a cheap regulator, the regulator can't react fast enough at high rpm and the voltage available to your ignition is effectively reduced and you develops a high speed miss.