I take it you were assigned to analyze this paper? It's not easy for me to follow, either, in part because there are a lot of terms that may be special to your field and/or your location. (I see the term "logging coupe" used only in Australia, for example.) But I think I figured out the parts you are asking about.
The way I read the bit you quoted, "Only 21% of all Greater Gliders detected during surveys were seen by both observers, indicating a low detection probability," is not that each person detects 21% of the GG's that the other person detected, but that 21% of the total number of GG's that were detected at all were detected by both. That is very different. For example, it is possible that A detected 100 GG's and B detected 21 of those, and no others. Then 21% of the detected GG's were detected by both, but A detected 100% of those that B detected, while B detected 21% of those detected by A. There are many other possibilities. There is no reason to assume both observers saw the same number.
But given the total number observed, you can determine the total number of detections, which was your question. If N were observed (after taking duplicate observations into account), then 0.21N were detected twice and 0.79N were detected once. The total number of detections was therefore 2*0.21N + 0.79N = 1.21N. (That makes sense, if you think about it.) This is closer to your colleague's 1.25 than to your 1.11.
Another point to make is that they didn't assume that 21% were seen twice; they carefully compared their observations to determine which were duplicates: "On completion of the survey the observers walked back along the transect line together, comparing observations on each individual seen. The two observers’ data were then combined to determine which gliders had been seen only by observer one or observer two, or by both observers. In almost all cases it was straightforward to determine whether each glider had been seen by observer one, observer two, or by both observers. Where it was not obvious, duplicate observations were determined by carefully comparing the coordinates collected by each observer combined with perpendicular distances of gliders from each sighting using GIS. Individual observers alternated between being observer 1 or observer 2." (section 2.3)
As for section 3.3, I think it says that 121 GG's were detected, not that there were 121 detections before eliminating duplicates: "the total set of 121 gliders detected". But if it were 121 detections, then I would say that there were 100 individual GG's detected, based on what I said above.
Finally, I'd say that "the apparent deficit of detections at distances <15 m" means that in theory they would expect the histogram to be decreasing everywhere, indicating that the closer it is, the easier it is to detect; the peak around 15 m suggests, as they say, that it is in fact harder to detect animals almost directly above you, because the numbers there are lower than expected (not low in an absolute sense, but compared to the model). Apparently their analysis (which I didn't try to examine) is based on a model (MRDS) that assumes that detection probability is inversely proportional to distance, or something like that; so they chose to fake it by essentially cutting out the region with 15 m of the path, and the region more than 75 m, so that the model can be applied to that region. (What I am curious about is the big notch around 50 m in the histogram, which they ignore; that seems more noticeable than the drop-off after 75 m.)
