והטור הרביעי תרשיש ושהם וישפה
In the listing of the stones on the חושן there is a difference between the last row and the other three. The last row is "תרשיש ושהם וישפה" There is a וי"ו before the second stone as well as the third. In the other three rows, the וי"ו appears only before the last stone. משך חכמה points out that the reason for this is as we find in קריאת שמע, that certain groups of words have the first word beginning with the same letter as the next word like "על לבבכם" and therefore must be very carefully differentiated. So, too, here תרשיש and שהם have the same problem. Therefore, in order to differentiate between the two, 'ח told Moshe "תרשיש ושהם" so he would not get mixed up.
The difficulty is, however, that in (פקודי (ל"ט:י"ג the list does not contain a וי"ו before שהם. Although משך חכמה does make mention of this fact he does not clearly indicate why that is. ר' ברוך אפשטין, in ברוך שאמר, gives an answer. In ה', תצוה is talking to משה. Therefore, it was important there to differentiate between the two so that there is no confusion. In פקודי, however, the Torah is merely giving a recount of events so it was not imperative to place a וי"ו in the middle.
One of the members of the חבורה where I heard this brought up an interesting point. At the beginning of שמות we seem to find a similar phenomenon. When listing the sons of יעקב a וי"ו is only used for the last name in each פסוק. Except for א:ד, where there is a וי"ו before נפתלי. It would seem that this is to differentiate between the נו"ן at the end of דן and the נו"ן at the beginning of נפתלי. However, here it seems only to be giving a recount and there is no one speaking to anyone. I do not know an answer to that problem.