I did a similar analysis a few months ago, which was focused more on general distinctions between keywords within a topic. This analysis is much more focused on the types of variations of keywords that we are seeing being grouped. These types of variations were categorised by, among others, jennifer slegg at the sem post.

The Five Types of Variations That I’ve Looked Into for This Analysis Are the Following

Initialisms/abbreviations. For example, comparing serps for [bbc] and [british broadcasting corporation]
Plurals. For example, [waffle maker] and [waffle makers].
Verb stems with and without suffixes. For example, [calculate], [calculated] and [calculating].
Keywords with and without punctuation. For Apparel and Clothing Manufacturers Email List example, [midnight’s children] and [midnights children]
Keywords with and without typos. For example, [heart rate monitor] and [heart rat monitor].
For each of these five categories, I put together a list of 50-100 keywords, along with a variation for each.

I Also Looked at How Common It is That Serps Are Exactly Identical

That is that the top ten results are the same DW Leads pages, in the same order. This showed an interesting pattern. There are only two categories with significant numbers of identical serps—punctuation and typos. In the case of keywords with and without punctuation, you are more likely to see identical serps (implying that google sees the pair of keywords as identical) if keyword volumes are grouped than if they are not. This is not a hard-and-fast rule though – there are still some ungrouped keywords which have identical serps.