Just wondering why you would lower the tip if it was the kitchen that was behind, and not the server?
Mabye it's a cultural thing?
Where I am from (the Netherlands) in a lot of restaurants you do not tip just the server. The served meal is a team effort, therefore tips are collected over the evening and at the end divided equally over the servers, cooks, people handling the dish washer, other kitchen staff etc. If, for example, I ordered a dessert without whipcream and the server brings me a dessert with whipcream it doesn't necessarily mean the server messed up the order, it could have happened that the cook didn't read the ordering system correctly. But whoever got confused/misread/screwed up, I would send the dessert back to the kitchen. And it would be a reason for me to adjust the tip. It also works the other way. We had a the superb evening at Jiko. We tipped generously, not just because Hassan was a great server and really exceeded our expectations, but also because the meals were perfectly prepared and the nice welcome and chat we had with the people receiving us and bringing us to our table. All of them together made a great evening.
But even if things go differently in the US and the tip only went to the girl who served our drinks/meals. Eventhough she was a nice girl and did her job, she could have informed us honestly and timely about delays in the kitchen. After the waiting took a lot longer than acceptable, I stood up from my table to find her to ask how much longer we would have to wait. I understand she didn't do that up front, why worry a guest when there might not be a reason for it. But at some point she must have realized our entrees were taking longer than normally. If she had come to us and said: 'fire -> evacuation, things take longer' I would have been understanding. Those kind of events are beyond everyone's control. Now that she only said 'kitchen running behind' and only after I asked, my thoughts were themed: 'bad planning'. Especially as the restaurant wasn't full, we came in around 5:30, left around 7, in our section at least 7 tables (for 4 people each) were unoccupied for as long as we were there. All I could see as a guest might have been different from the point of view of the restaurant, but at that time it influenced my dining experience.
For the record, we ate there with 2 adults, we are talking about 15 dollar instead of 18 dollar tip.