I just don't understand GMATPrep Verbal

Even I had the same question. We cannot get the answer from those who are consistent because they cannot understand our position. This is what I found after some analysis.
1) Concentration plays a big role. If you are fresh, you will get good score.
2) Experimentation. If you try something new, your score tends to swing.
3) There are also swings based on question type. Like I was not good with inference questions. Hence, questions like those tend to affect my score.

Finally, the only strategy is found is to devise a very concrete plan and stick to it (like a machine, no matter what the consequences are). It helps me to get rid of pressure and will make you (at least) feel consistent.

I am definitely in the same boat as you guys. During an intense 3 month study schedule (roughly 20-30 hours a week) I got anywhere from 40-47 in verbal which I thought was great but was also surprised to see the wide swings (for example I would get a 40 on one GMAT prep one week and get a 46 or 47 the next and then back to a 40 the following week).

And guess what - I took the actual GMAT today and got only a 36 in verbal. I wanted to shoot myself! IMHO the actual GMAT verbal is much harder than the GMATPrep. The CRs are much more confusing (sometimes you feel the right answer could be one of 3) and I had close to 100% hit rate on those in practice and even the SC seems like it's testing other concepts than what I prepared for. Was very humbled by this tough experience today so we're all in the same boat.

If anyone can share some golden advice on how to improve and keep consistency, I am sure everyone here will be ever so grateful

I concur on verbal. I got the same verbal score on practice I was getting after a year of studying.

My GMAT prep score swung wildly, and did not always make sense. At one point GMAT prep gave me a 42v even though I only got 2 or 3 wrong!

As to improvement with study, my CR and RC have always been good, but I was able to substantially improve my SC over about 2 months of intense work. My verbal average went from the low 40s to the mid 40s (with a 46V on the actual gmat).

One important thing to note: the verbal questions on gmat prep are the only questions that are similar to the gmat! Those on m-gmat and others do not match the feel of real gmat verbal questions.