So far as I know the relatively low (within normal) TSH while having relatively low (within normal) free T3 would be due to relatively low production of TRH, the hormone that stimulates the pituitary to make TSH, or relatively low production of TSH for given amount of TRH.
Broadly, relatively low TRH would be from energy-sensing, or could be from asynchronized or disturbed circadian rhythm (the "clocks" within the body are not necessarily synchronized though they should be, and when not, there's metabolic disturbance.)
There are various inputs related to energy-sensing, including leptin, neuropeptide-Y, and agouti-related protein. None of this helps to know really except for these being reasons why dieting and post-dieting there are effects on thyroid function.
Stress or high cortisol can decrease TRH and TSH.
If your values remained, I'd suppose the reasons would be any of, continued sensing of low energy state (not having returned to a point of bodyfat where your body can maintain high metabolic rate), disturbed circadian rhythm, high stress, or individual variation within the normal range.
What it wouldn't be a sign of would be a problem with the thyroid itself or of iodine deficiency.
Loss of body weight, and particularly body fat, tends to depress thyroid function. It's good news that your TSH is so good (low.)