I gather your child is nine years old?
Yes, the possible standard score for that subtest component at that age tops out at 134. The Sentence Combining component tops out at 144, and the whole Sentence Composition subtest has a maximum obtainable score of 149.
The short answer is that these are the limits of the instrument. Statistically, one doesn't often need standard scores higher than that. (BTW, the same issue exists for the floor of this particular subtest: the lowest obtainable component score for SB at this age is 55, and the lowest possible subtest score is 56.) Test designers have to balance range, sensitivity, reliability, psychometric robustness, etc. with usability. One could design a test with a higher ceiling, but it would probably need more items. Since Sentence Composition is administered in its entirety to every student in its age/grade range (age 6+/grade 1+), the additional length would benefit less than 1% of students, but would increase factors related to test length and fatigue for 100% of students. This is not as much of a factor for subtests with more traditional number-of-items-incorrect-in-a-row discontinue critera as it is for subtests with item sets, like SC and Reading Comprehension.
