Because you are casting between Array<Float> and Array<Array<Float>> like in following sample?
var metrics:Array<Array<Float>> = [[1,1,1,478,526,559,559,561,561,544,469,734,734,734,770,1017,1036,1131,1255,1338,679,934],[1,1,1,478,526,559,559,561,561,544,469,734,734,734,770,1017,1036,1131,1255,1338,679,934],[1,1,1,478,526,559,559,561,561,544,469,734,734,734,770,1017,1036,1131,1255,1338,679,934],[489,489,489,1,65,99,99,101,100,139,31,271,271,271,272,648,667,762,1004,969,687,942],[528,528,528,65,1,35,35,47,94,187,71,319,319,319,266,602,621,716,955,923,682,936],[562,562,562,99,35,1,1,55,128,221,104,285,285,285,322,568,587,682,921,890,814,1069],[562,562,562,99,35,1,1,55,128,221,104,285,285,285,322,568,587,682,921,890,814,1069],[564,564,564,101,47,47,47,1,108,222,106,315,315,315,280,598,617,711,951,919,695,950],[571,571,571,101,95,128,128,108,1,161,113,255,255,255,256,472,652,747,986,954,590,845],[551,551,551,136,183,217,217,219,157,1,126,389,389,389,390,744,763,857,1066,1065,590,845],[471,471,471,31,78,112,112,114,113,130,1,335,335,335,285,618,637,732,971,939,700,955],[743,743,743,271,258,292,292,272,256,393,277,1,1,1,37,379,398,492,732,700,530,785],[743,743,743,271,258,292,292,272,256,393,277,1,1,1,37,379,398,492,732,700,530,785],[743,743,743,271,258,292,292,272,256,393,277,1,1,1,37,379,398,492,732,700,530,785],[776,776,776,276,263,325,325,276,260,397,281,34,34,34,1,242,430,525,764,733,534,789],[1031,1031,1031,488,613,579,579,607,472,758,629,261,261,261,238,1,390,485,718,637,638,726],[1039,1039,1039,667,621,587,587,615,651,766,637,396,396,396,432,390,1,165,425,621,925,1180],[1133,1133,1133,762,716,682,682,710,745,861,732,491,491,491,527,485,169,1,389,713,1020,1274],[1263,1263,1263,1007,959,925,925,953,988,1073,975,734,734,734,770,721,414,400,1,791,1263,1517],[1341,1341,1341,969,923,890,890,918,953,1068,939,699,699,699,735,634,617,710,791,1,1227,1385],[692,692,692,695,682,715,715,695,587,592,700,530,530,530,531,641,926,1021,1260,1229,1,256],[947,947,947,950,936,970,970,950,842,847,955,785,785,785,786,733,1181,1276,1515,1428,256,1]];
Tsp(null,metrics);
var metrics:Array<Float> = [1,1,1,478,526,559,559,561,561,544,469,734,734,734,770,1017,1036,1131,1255,1338,679,934];
Tsp(null,cast metrics); // Uncaught exception: Can't cast hl.types.ArrayBytes_Float to hl.types.ArrayObj