Here are the first 25 COCA results for Adjective + in understanding (the number on the right is the number of hits for a sequence). In all of these the adjective preceding the prepositional phrase characterizes instrument which leads to understanding rather than characterizing some person's ability at understanding.
1 HELPFUL IN UNDERSTANDING THE 26
2 USEFUL IN UNDERSTANDING THE 24
3 INTERESTED IN UNDERSTANDING THE 23
4 IMPORTANT IN UNDERSTANDING THE 15
5 HELPFUL IN UNDERSTANDING HOW 8
6 INTERESTED IN UNDERSTANDING HOW 5
7 IMPORTANT IN UNDERSTANDING AND 4
8 CRITICAL IN UNDERSTANDING THE 4
9 INVOLVED IN UNDERSTANDING THE 4
10 USEFUL IN UNDERSTANDING HOW 4
11 IMPORTANT IN UNDERSTANDING HOW 3
12 HELPFUL IN UNDERSTANDING GLOBAL 3
13 CRUCIAL IN UNDERSTANDING THE 3
14 INTERESTED IN UNDERSTANDING THEIR 3
15 INTERESTED IN UNDERSTANDING WHAT 3
16 INVOLVED IN UNDERSTANDING OR 3
17 USEFUL IN UNDERSTANDING WHY 3
18 BENEFICIAL IN UNDERSTANDING THE 2
19 ESSENTIAL IN UNDERSTANDING THE 2
20 CRITICAL IN UNDERSTANDING HOW 2
21 INTERESTED IN UNDERSTANDING AND 2
22 HELPFUL IN UNDERSTANDING WHY 2
23 HELPFUL IN UNDERSTANDING WHAT 2
24 NECESSARY IN UNDERSTANDING WHAT 2
25 PIVOTAL IN UNDERSTANDING THE 2
Now see the results for Adjective + at understanding. In all of these, the adjective characterizes a person's ability to understand.
1 GOOD AT UNDERSTANDING THE 4
2 SUCCESSFUL AT UNDERSTANDING MICROBIAL 1
3 SLOW AT UNDERSTANDING AND 1
4 POOR AT UNDERSTANDING ECOLOGICAL 1
5 INCOMPETENT AT UNDERSTANDING HOW 1
6 GOOD AT UNDERSTANDING WHY 1
7 GOOD AT UNDERSTANDING THEIR 1
8 GOOD AT UNDERSTANDING TECHNICAL 1
9 GOOD AT UNDERSTANDING RIDDLES 1
10 GOOD AT UNDERSTANDING OR 1
11 GOOD AT UNDERSTANDING HOW 1
12 GOOD AT UNDERSTANDING GLOBAL 1
13 BETTER AT UNDERSTANDING DIFFERENT 1
14 BETTER AT UNDERSTANDING A 1
15 BAD AT UNDERSTANDING WHAT 1
16 BAD AT UNDERSTANDING HOW 1
17 ADEPT AT UNDERSTANDING THEIR 1
18 ADEPT AT UNDERSTANDING AND 1
It wouldn't be safe, however, to assume that this pattern holds for any given adjective + PP[pres.ppl] combination. When we search for skilled + (in/at) + V.pres.ppl we find 109 hits for skilled at V-ing and 173 hits for skilled in V-ing. Not much of a discrepancy as compared to the clear differentiation between in understanding and at understanding.