Please use the correct terminology. Each "Scenario" in a load test has its own load pattern. This answer assumes that there are 5 test cases A to E.
The precise way load tests start test cases is not defined but the documentation is quite clear. Additionally the load test wizard used when initially creating a load test has good descriptions of the test mix models.
Load tests also make use of random numbers for think times and when choosing which test to run next. This tends to mean the final test results show counts of test cases executed that differ from the desired percentages.
My observations of load tests leads me to believe it works as follows. At various times the load test compares the number of tests currently executing against the number of virtual users that should be active. These times are when the load test's clock ticks and a step load pattern changes, also when a test case finishes. If the comparison shows more virtual users than test cases being executed then sufficient new tests are started to make the numbers equal. The test cases are chosen to match the desired test mix, but remember that there is some randomization in the choice.
Your step pattern is initially 10, step by 10 every 10 sec to a maximum of 300. Maximum users should be after (10 seconds per step)*(300 users)/(10 users per step) = 300 seconds = (5 minutes). The run duration of 10 minutes means 5 minutes ramp up then 5 minutes steady at max users.
For the final paragraph of your question. Given the same percentages but an constant user count of 100 then you would expect the initial number of each test case to be close to the percentages. Thus 10 of A, 65 of B, 9 of C, 8 of D and 8 of E. When any test case completes visual studio will choose a new test case attempting to follow the test mix model, but, as I said earlier, there is some randomization in the choice.
