This week's Parsha describes the various families of the Leviyim and their specific functions. The Torah states about the family of Kehos that they should be counted "מבן שלשים שנה ומעלה ועד בן חמשים שנה כל בא לצבא לעשות מלאכה באהל מועד," "from thirty years of age and up, until fifty years of age, all who (are fit to) come to do work in the Ohel Moed" (במדבר ד':ג'). Commenting on this Posuk, the Netziv points out that this is the only time in the series of Pesukim about the Leviyim that the Torah writes "בא לצבא," and not "הבא לצבא". This missing letter "ה", says the Netziv, changes the whole meaning of the word.
The word "הבא" means "all who come"; "בא," however, means "all who are fitting or suitable (to work in the Ohel Moed) can come." An example of this usage of the word "בא" can be found in Yechezkel (ט"ז:ט"ז), where the Posuk states "לא באות ולא יהיה." There, Rashi comments on the phrase לא באות, saying that in this context it means "not fitting to be there." The Netziv then cites another source from the Midrash, which points out that the number of people between the ages of thirty and fifty among Bnai Kehos was proportionately much less in relationship to their total as compared to that number among Bnai Gershon and Bnai Merari. The Midrash suggests that the reason for this was that the danger involved in their job (transporting the Aron) was so great that some were not suitable for it and therefore died. However, the Netziv is quick to point out that the counting of Bnai Kehos was recorded before they were assigned to do their job. Therefore, he suggests that by interpreting the word בא to mean "all who are fitting to come," one can now conclude that only the people between thirty and fifty who were suitable for the job were counted, and hence the lower total number.
The Netziv further comments on this Posuk, noting that it is also the only place where the word מלאכה (and not עבודה) is used in the Torah to describe the jobs of the Leviyim. Since the job of the Bnai Kehos was to carry the Aron, one could infer from this Posuk that there is a prohibition of מעביר ד' אמות ברשות הרבים, carrying four Amos in a public domain on Shabbos, since what was done in the Mishkan is forbidden on Shabbos, and categorized as a מלאכה. However, this Halacha, when discussed in the Gemara in Shabbos (דף צ"ו:), is not stipulated as being derived directly from the Torah, but rather is learned מפי השמועה, through the Oral Mesorah. Therefore, the Netziv explains, when our Posuk talks about Bnai Kehos doing a מלאכה, it does not refer to carrying the Aron, but rather to their other job of making new vessels in the Bais HaMikdash. The phrase "בא לצבא," however, refers to those who have the job of transporting the Aron, as explained above.