increasing the number of validations means, that you divide your dataset into more parts. Since only one part is used for testing and the regression is learned on all other parts, this means you increase the total number of training examples. And hence the dependency is linearly constructed the linear regression captures it more accurate and the error is reduced on the test set.
(This depends on the property of the linear regression to converge with an infinite number of examples constructed with linear dependency into this dependency, if only random error is added.)
But since the test sets now decrease in size, there are more sets consisting of examples with very bad examples, causing a high error and more with very easy examples with low error. So the variance of the error increases, thats why the standard deviation of the RMS increases.