Then something is wrong and I screwed up with the data, as the listings I have show DD historically coming out a week ahead of the Hulk. Was 181 as anomaly and came out a week early or something?
The week of release for many books changed over time. Looking at only a small sample, then applying those conclusions to other periods of time, can lead to inaccurate assumptions. I have done data collection and analysis for nearly every Marvel and DC book (and many other publishers) across all time periods. Not all periods have the same amount of release date data available. For example, Marvel from Aug 1969 to Apr 1972 is a mess for many reasons. However, the availability of data beyond May 1972 is in quite good shape.
Regarding DD and Hulk:
Up until July 1972 (DD89/Hulk153), Daredevil did come out the week before Hulk.
However, from Aug 1972 (DD90/Hulk154) to Sep 1975 (DD125/Hulk191), Hulk and Daredevil shipped together on the same day. The only exceptions to this were the months in which Daredevil was not released at all (Feb 1974 & Apr 1974). Obviously DD115 and Hulk181 fall during this time period.
Following this period, beginning with books dated Oct 1975 (DD126/Hulk192) - Hulk shipped two weeks after DD.
Also, for this period Marvel's cover dates did not align precisely with months of release.
For example all books cover dated November did not got on sale in August. A few actually went on sale the last week of July. That's why both Hulk 180 and 181 went on sale in July, the 2nd and 30th respectively. From 1972 through then end of the 1970s, DD was consistently in the first week of Marvel releases with a given cover month, though it did have several changes in frequency which caused it to miss months altogether. FF was consistently in the last week of releases for a given cover month.